UC-NRLF 


»B    30b   52? 


GIFT  OF 
Dr.   Horace  Ivie 


EDUCATION  DEFT 


PLYMPTON'S  PARKER'S  PHILOSOPHY, 


A  SCHOOL  COMPENDIUM 

NATURAL   AND    EXPERIMENTAL 

PHILOSOPHY: 

EMBRACING     THE    ELEMENTARY     PRINCIPLES     OF 


MECHANICS,  HYDROSTATICS,  HYDRAULICS    I&tipFICfe,  ACOFS.TTTS, 
PYRONOMICS,    OPTICS,    ELECTRICITY-,'  GALVANISM,  'MAGNETI6li,' 
ELECTRO-MAGNETISM, 


CONTAINING   ALSO  ,A    DESCRIPTION    OF    THE 

STEAM   AND    LOCOMOTIVE    ENGINES, 

.jfllND    OF    THE 

ELECTRO-MAGNETIC  TELEGEAPH. 
BY 

RICHARD  GREEN  PARKER,  A.M., 

AUTH0R    OP    "AIDS    TO    ENGLISH    COMPOSITION,"    A'  SERIES    OF    "SCHOOL    READERS,"    ETC. 


Delectando  pariier  que  monendo. 
Prodesse  quam  conspici. 


A    NEW    EDITION,    REVISED    AND    ENLARGED, 

BY  GEO.  W.  PLYMPTON,  A.M., 

PROFESSOR    OF    PHYSICAL    SCIENCE,    BROOKLYN    POLYTECHNIC     INSTITUTE. 


NEW    YORK:       - 

COLLINS   &   BROTHER,    PUBLISHERS, 
370    BROADWAY. 


G1FTOF  -TE>   -2 


'  "P 

CONTENTS. 


DIVISIONS  OF  THE  SUBJECT,  .  .  .  .  .  «.  .17 
OP  MATTER  AND  ITS  PROPERTIES,  *  .  ;  .  .  19 

OF  GRAVITY,  .  .  .  .-*1  •  ||  ....  33 
MECHANICS,  on  THE  LAWS  OF  MOTION,  .  .  .  .  41 
THE  MECHANICAL  POWERS,  .  .  .  ••  .  '  •  •  •  70 
REGULATORS  OF  M^oSt,  ....  ^  ..  100 

S,   V/  «  ?"«"    ;          .,         .          •          •  •          •          .108 

-;    .t  A,      ?\       . ' 128 

? " "  \*       .       .       .       .  .       .       .138 

ACOUSTICS,       .        .        .        ...        .        .    "  '.* •'-    .'       .        173 

PYltONOMICS, -        .      185 

THE  STEAM-ENGINE, •       •  196 

OPTICS,         .        .        .        .        .*      .        .        .        ,        .        .210 

ELECTRICITY, 258 

GALVANISM,  OR  VOLTAIC  ELECTRICITY,  .....  283 

MAGNETISM, .  298 

ELECTRO  MAGNETISM,        . 308 

THE  ELECTROMAGNETIC  TELEGRAPH,         ....  319 

THE  ELECTROTYPE  PROCESS, 331 

MAGNETO-ELECTRICITY,          .        .        .        .        .       .       .  332 

THERMO-ELECTRICITY, 334 

ASTRONOMY,                    , 335 

APPENDIX, .        ...        .  403 


The  Index  at  the  close  of  the  volume,  being  full  and  comprehensive,  will  be 
found  more  convenient  for  reference. 

Entered  according  to  Act  of  Congress,  in  the  year  1871,  by 

.    COLLINS  &  BROTHER, 
In  tho  Office  of  the  Librarian  of  Congress,  at  Washington. 

EDUCATION  DEPT 


P  E  E.  F  A  O  E 

TO  THE 

REVISED    AND    ENLARGED    EDITION. 


THE  favor  with  which  this  book  has,  from  its  first  appearance, 
been  received  by  the  teachers  of  this  country,  has  induced  the  pub- 
lishers to  offer  yet  another  edition  to  the  schools  of  the  United  States. 

It  is  presented  as  a  revision  and  an  enlargement  of  the  previous 
edition. 

The  revision  of  the  book  has  led  to  such  corrections  of  the  text  of 
the  older  work  as  the  recent  progress  in  physical  science  demanded. 
This  has  been  accomplished  without  changing  the  numbering  of 
the  paragraphs  or  their  distribution  on  the  pages.  Where  a  more 
extended  correction  seemed  necessary  than  this  plan  permitted,  the 
reader  has  been  referred  by  note  to  the  Appendix  for  the  supple- 
mentary portion. 

It  was  deemed  an  exceedingly  desirable  object  by  the  publishers 
that  the  new  work  should  be  presented  in  such  shape  that,  when 
introduced  to  classes  using  the  old  edition,  the  exchange  might  be 
effected  with  the  least  possible  inconvenience  to  teacher  and  pupil. 

The  principal  emendations  have  been  made  in  the  subjects  of 
Mechanics,  Heat,  Hydrodynamics,  and  Optics.  In  Mechanics  par- 
ticularly, the  progress  of  ideas  within  a  short  period  demands  that 
the  rudimentary  conceptions  of  Force,  Power,  and  work  in  the  mind 
of  the  learner  should  be  more  sharply  defined.  The  first  paragraphs 
of  the  Appendix,  giving  the  distinction  between  these  terms,  and 
also  introducing  the  term  Energy,  have  been  prepared  in  accordance 
with  this  demand. 

The  mechanical  theory  of  Heat, — the  practical  relation  of  Hydro- 
statics and  Hydraulics  to  Mechanical  Engineering, — the  later  uses 
of  compressed  air,  and  the  theory  of  the  Spectroscope,  have  received 
a  due  share  of  space  in  the  additional  pages. 

A  large  number  of  new  illustrations  have  been  added,  which,  it  is 
hoped,  will  aid  the  necessarily  concise  Appendix. 

GEO.  W.  PLTMPTON. 

POLYTECHNIC  INSTITUTE,  December,  1871. 

934218 


INTRODUCTION. 


THE  term  Philosophy  literally  signifies,  the  loye  of 
wisdom ;  but,  as  a  general  term,  it  is  used  to  denote  an 
explanation  of  the  reason  of  things,  or  an  investigation 
of  the  causes  of  all  phenomena,  both  of  mind  and  of 
matter. 

When  applied  to  any  particular  department  of  knowl- 
edge, the  word  Philosophy  implies  the  collection  of  general 
laws  or  principles,  under  which  the  subordinate  facts  or 
phenomena  relating  to  that  subject  are  comprehended. 
Thus  that  branch  of  Philosophy  which  treats  of  God,  his 
attributes  and  perfections,  is  called  Theology;  that  which 
treats  of  the  material  world  is  called  Physics,  or  Natural 
Philosophy;  that  which  treats  of  man  as  a  rational  being 
is  called  Ethics,  or  Moral  Philosophy;  and  that  which 
treats  of  the  mind  is  called  Intellectual  Philosophy,  or 
Metaphysics. 

The  natural  division  of  all  things  that  exist  is  into 
body  and  mind — things  material  and  immaterial,  spiritual 
and  corporeal.  Physics  relates  to  material  things,  Meta- 
physics to  immaterial.  Man,  as  a  mere  animal,  is  includ- 
ed in  the  science  of  Physics  ;  but,  as  a  being  possessed  of 
a  soul,  of  intellect,  of  the  powers  of  perception,  conscious- 
ness, volition,  reason,  and  judgment,  he  becomes  a  sub- 
ject of  consideration  in  the  science  of  Metaphysics. 

All  material  things  are  divided  into  two  great  classes, 
called  organized  and  unorganized  matter.  Organized 
matter  is  that  which  is  endowed  with  organs  adapted  to 
the  discharge  of  appropriate  functions,  such  as  the  mouth 
and  stomach  of  animals,  or  the  leaves  of  vegetables.  By 
means  of  such  organs  they  enjoy  life.  Unorganized  mat- 
ter, on  the  contrary,  possesses  no  such  organs,  and  is  con- 


INTRODUCTION.  V 

sequently  incapable  of  life  and  voluntary  action.  Stones, 
the  various  kinds  of  earth,  metals,  and  many  minerals, 
are  instances  of  unorganized  matter.  Fossils,  that  is, 
substances  dug  out  of  the  earth,  are  frequently  instances 
of  a  combination  of  organized  and  unorganized  matter. 
Unorganized  matter  also  enters  into  the  composition  of 
organized  matter.  Thus,  the  bones  of  animals  contain 
lime,  which  by  itself  is  unorganized  matter. 

Physical  Science,  or  Physics,  with  its  subdivisions  of 
Natural  History  (including  Zoology,  Botany,  Mineralogy, 
Conchology,  Entomology,  Ichthyology,  &c.)  and  Natural 
Philosophy,  including  its  own  appropriate  subdivisions, 
embraces  the  whole  field  of  organized  and  unorganized 
matter. 

The  term  Natural  Philosophy  is  considered  by  some 
authors  a's  embracing  the  whole  extent  of  physical  science, 
while  others  use  it  in  a  more  restricted  sense,  including 
only  the  general  properties  of  unorganized  matter,  the 
forces  which  act  upon  it,  the  laws  which  it  obeys,  the 
results  of  those  laws,  and  all  those  external  changes  which 
leave  the  substance  unaffected.  It  is  in  this  sense  that 
the  term  is  employed  in  this  work. 

Chemistry,  on  the  contrary,  is  the  science  which  inves- 
tigates the  composition  of  material  substances,  the  inter- 
nal changes  which  they  undergo,  and  the  new  properties 
Which  they  acquire  by  such  changes.  The  operations  of 
chemistry  may  be  described  under  the  heads  of  Analysis 
or  decomposition,  and  Synthesis  or  combination. 

Natural  Philosophy  makes  us  acquainted  with  the  con- 
dition and  relations  of  bodies  as  they  spontaneously  arise, 
without  any  agency  of  our  own.  Chemistry  teaches  us 
how  to  alter  the  natural  arrangement  of  elements  to  bring 
about  some  particular  condition  that  we  desire.  To  ac- 
complish these  objects  in  both  of  the  departments  of 
science  to  which  we  refer,  we  make  use  of  appliances 
called  philosophical  and  chemical  apparatus,  the  proper 
use  of  which  it  is  the  office  of  Natural  Philosophy  and 
Chemistry  respectively  to  explain.  All  philosophical 
knowledge  proceeds  either  from  observation  or  experi- 
ment, or  from  both.  It  is  a  matter  of  observation  that 
water,  by  cold,  is  converted  into  ice ;  but  if,  by  means  of 
freezing  mixtures,  or  evaporation,  we  actually  cause  water 
to  freeze,  we  arrive  at  the  same  knowledge  by  experiment. 


VI  INTRODUCTION. 

By  repeated  observations,  and  by  calculations  based  on 
such  observations,  we  discover  certain  uniform  modes  in 
which  the  powers  of  nature  act.  These  uniform  modes 
of  operation  are  called  laws; — and  these  laws  are  general 
or  particular  according  to  the  extent  of  the  subjects 
which  they  respectively  embrace.  Thus,  it  is  a  general 
law  that  all  bodies  attract  each  other  in  proportion  to  the 
quantity  of  matter  which  they  contain.  It  is  a  particular 
law  of  electricity  that  similar  kinds  repel  and  dissimilar 
kinds  attract  each  other.  , 

The  collection,  combination,  and  proper  arrangement 
of  such  general  and  particular  laws,  constitute  what  is 
called  Science.  Thus,  we  have  the  science  of  Chemistry, 
the  science  of  Geometry,  the  science  of  Natural  Philo- 
sophy, &c. 

The  terms  art  and  science  have  not  always  been  em- 
ployed with  proper  discrimination.  In  general,  an  art  is 
that  which  depends  on  practice  or  performance,  while 
science  is  the  examination  of  general  laws,  or  of  abstract 
and  speculative  principles.  The  theory  of  music  is  a 
science ;  the  practice  of  it  is  an  art. 

Science  differs  from  art  in  the  same  manner  that 
knowledge  differs  from  skill.  An  artist  may  enchant  us 
with  his  skill,  although  he  is  ignorant  of  all  scientific 
principles.  A  man  of  science  may  excite  our  admiration 
by  the  extent  of  his  knowledge,  though  he  have  not  the 
least  skill  to  perform  any  operation  of  art.  When  we 
speak  of  the  mechanic  arts,  we  mean  the  practice  of  those 
vocations  in  which  tools,  instruments  and  machinery,  are 
employed.  But  the  science  of  Mechanics  explains  the 
principles  on  which  tools  and  machines  are  constructed, 
and  the  effects  which  they  produce.  Science,  therefore, 
may  be  defined,  a  collection  and  proper  arrangement  of 
the  general  principles  or  leading  truths  relating  to  any 
subject;  and  there  is  this  connection  between  art  and 
science,  namely — "  A  principle  in  science  is  a  rule  of  art." 


NATURAL  PHILOSOPHY. 


DIVISIONS    OP    THE    SUBJECT. 

„_     .         1.  NATURAL  PHILOSOPHY,  or  PHYSICS,  is  the 

\\  hat  is         .  , 

Natural    science  which  treats  of  the  powers,  properties  and 

Philoso-    mutual  action  of  natural  bodies,  and  the  laws  and 

operations  of  the  material  world. 

1.  Some  of  the  principal  branches  of  Natural  Philosophy  are 
Mechanics,  Electricity, 

Pneumatics,  Galvanism, 

Hydrostatics.  Magnetism, 

Hydraulics,  Electro-Magnetism, 

Acoustics,  Magneto-Electricity. 

Pyronomics,  Astronomy. 

Optics, 

NOTE. — This  list  of  branches  might  be  considerably  enlarged,  but  per- 
haps a  rigid  classification  would  rather  suggest  the  omission  of  some  ol 
tbem,  as  pertaining  to  the  department  of  chemistry. 

What  is        ^'  MECHANICS.  — Mechanics  is  that  branch  of 
Mechan-    Natural  Philosophy  which  relates  to  motion  and 
the  moving  powers,  their  nature  and  laws,  with 
their  effects  in  machines.  • 

4.  Mechanics  is  generally  considered  under  two  division*,  culled 

Stfities  uud  Dynamics. 


18  NATURAL    PHILOSOPHY. 

5.  The  word  Statics  is  derived  from  a  Greek  word  implying  test 
and  it  is  applied  to  that  department  of  mechanics  which  treats  of 
the  properties  and  laws  of  bodies  at  rest. 

6.  Dynamics,   from    a   Greek   word   signifying  power  or  force 
treats-  q£  $he  prapeYtie^  |vn>l  i&ws  of  bodies  in  motion. 

SJ  "V,,^  7  ''I,0     'I  C^S  "y? 

I.  Pneumatics  treats  of  t,h<^  mechanical  properties  and  effecta 
o^)^,gr\aii^\KlEaiIjapj3didiiL  ^ill.ec^'elastic  fluids  or  gases. 

8.  Hydrostatics  treats  of  the  gravity  and  pressure  of  fluids  in 
a  state  of  rest. 

9.  Hydraulics  treats  of  fluids  in  motion,  and  of  the  instru- 
ments and  machines  by  which  their  motion  is  guided  or  con- 
trolled. 

10.  Acoustics  treats  of  the  laws  of  sound. 

II.  Pyronomics  treats  of  the  laws  and  effects  of  heat. 

12.  Optics  treats  of  light,  color  and  vision. 

13.  Electricity  treats  of  an  exceedingly  subtle  agent,  jailed 
the  electric  fluid. 

14  Galvanism  (sometimes  called  chemical  f  l.ectricity)  is  a 
branch  of  Electricity. 

15.  Magnetism  treats  of  the  properties  and  effects  of  the 
magnet  or  loadstone. 

16.  Electro-Magnetism  treats  of  magnetism  inc  uced  by  elec- 
tricity. 

17.  Magneto-Electricity  treats  of  electricity  indu-sed  by  mag- 
netism. 

18.  Astronoiny  treats  of  the  heavenly  bodies,  —  the  sun,  moon. 
stars,  planets,  comets. 

19.  The   agents   whose   effects   or  operations   are   described    in 
Natural  Philosophy  are  divided  into  two  classes,  called  respectively 
Ponderable  and  imponderable  Agents. 

NOTE.  —  Some  writers  on  Philosophy  have  suggested  a  different  classi- 
fication, into  Bodies  and  Agents,  calling  bodies  ponderable,  and  agenta  »/»- 
ponderable. 


20.  Ponderable  agents  are  those  which  have  weight,  as 
air,  steam. 

21.  Imponderable  agents  are  those  which  have  no  weight  sueli 
as  light  heat,  magnetism  and  electricity. 


OF  MATTEE   AND   ITS   PEOPEETIES.  19 

What  is        22.  MATTEE. — Matter  is  the  general  name  of 
Matter?    everything  that  occupies  space. 

23.  Matter  exists  in  three  different  states  or  forms — 
namely,  in  the  solid,  liquid,  and  gaseous  forms. 

24.  Matter  exists  in  a  solid  form  when  the  particles  of  which  it  is 
composed  cohere  together.     The  different  degrees  of  cohesion  which 
different  bodies  possess  causes  them  to  assume  different  degrees  of 
hardness. 

25.  Matter  exists  in  a  liquid  state  when  the  component  parts  do 
not  cohere  with  sufficient  force  to  prevent  their  separation  by  the 
mere  influence  of  their  weight.     The  surface  of  a  fluid  at  rest  always 
conforms  itself  to  the  shape  of  the  portion  of  the  earth's  surface 
over  which  it  stands. 

26.  Matter  exists  in  a  gaseous  or  aeriform  state  when  the  par- 
ticles of  which  it  is  composed  have  a  repulsion  towards  each  other 
which  causes  them  to  separate  with  a  power  of  expansion  to  which 
there  is  no  known  limit.     Of  this,  smoke  presents  a  familiar  in- 
stance.    As  it  ascends  it  expands,  the  particles  repelling  each  other 
until  they  become  wholly  invisible. 

NOTE.  — The  word  aeriform  means,  m  the  form  of  air. 

27.  The  vesicular  form  of  matter  is  the  form  in  which  we  see  it 
in  clouds.     It  consists  of  very  minute  vesicles,  resembling  bubbles, 
arid  it  is  the  state  into  which  many  vapors  pass  before  they  assume 
a  fluid  condition. 

28.  Some  substances  are  capable,  under  certain  conditions,  of 
assuming  all  these  different  forms.     Water,  for  instance,  is  solid  in 
the  form  of  ice,  fluid  as  water,  in  the  gaseous  state  when  converted 
into  steam,  and  vesicular  in  the  form  of  clouds. 

29.  All  matter,  whether  in  the  solid,  liquid,  gaseous,  or  vesicular 
form,  is  either  simple  or  compound  in  its  nature.     But  this  consider- 
ation of  matter  pertains  more  properly  to  the  science  of  chemistry. 
It  is  proper,  however,  here  to  explain  what  is  meant  by  a  simple  01 
homogeneous  and  a  compound  or  heterogeneous  substance. 

30.  All  matter  is  composed  of  very  minute  particles  or  atoms 
united  together  by  different  degrees  of  cohesion.     When  all   the 
atoms  are  of  the  same  kind,  the  body  is  a  simple  or  homogeneous 
substance.     Thus,  for  instance,  pure  iron,  pure  gold,  &c.,  consists 
of  very  minute  particles  or  atoms,  all  of  which  are  pure  iron  or 
pure  gold.     But  water,  and  many  other  substances,  are  compound 
substances,  composed  of  atoms  of  two  or  more  different  substances, 
Combined  by  chemical  affinity. 

NOTE.  —  The  ancient  philosophers  supposed  that  all  material  substance* 
*ere  composed  of  Fire,  Air,  Earth  arid  AVater,  and  these  four  substanow 
•*re  called  the  f^ur  elements,  because  they  were  supposed  to  be  the  fiuiHt 

1* 


20 


NATUKAL   PHILOSOPHY. 


substances  of  which  all  things  are  composed.  But  modern  science  has 
shown  that  not  one  of  these  is  a  simple  substance.  Water,  for  instance, 
is  composed  of  two  invisible  gases,  called  Hydrogen  and  Oxygen,  united  in 
the  proportion  of  one  part,  in  weight,  of  hydrogen  to  eight  of  oxygen  ;  or, 
by  measure,  one  part  of  oxygen  to  two  of  hydrogen.  In  like  manner  air, 
or,  rather,  what  the  ancients  understood  by  air,  is  composed  of  oxygen 
mixed  with  another  invisible  gas,  called  nitrogen  or  azote,  m  the  proportion 
of  seventy-two  pa  rts  of  the  latter  to  twenty-eight  of  the  former. 

The  enumeration  of  the  elementary  substances,  which,  either  by  them- 
selves or  in  union  with  one  another,  make  up  the  material  world,  properly 
belongs  to  the  science  of  Chemistry.  As  this  work  may  fall  into  the  hands 
of  some  who  will  not  find  the  information  elsewhere,  a  list  of  the  simple 
substances  or  elements  is  herejpresented,  so  far  as  modern  science  has  in- 
vestigated them.  They  are  sixty-three  in  number,  forty-nine  of  which  are 
metallic,  and  fourteen  are  non-metallio. 

The  forty-nine  metals  are 


Gold, 

Silver, 

Iron, 

Copper, 

Tin, 

Mercury, 

Lead, 

Zinc, 

Nickel, 

Cobalt, 

Bismuth, 

Platinum, 

Antimony, 

Arsenic, 


Manganese, 

Cadmium, 

Uranium, 

Palladium, 

Ehodium, 

Iridium, 

Osmium, 

Titanium, 

Cesium, 

Tungsten, 

Molybdenum. 

Vanadium, 

Chromium, 


The  non-metallic  elements  are 

Oxygen,  Sulphur, 

Hydrogen,  Phosphorus, 

Nitrogen,  Carbon, 

Chlorine, 


Potassium, 

Sodium, 

Lithium, 

Barium, 

Strontium, 

Calcium, 

Magnesium, 

Aluminum, 

Glucinum, 

Yttrium, 

Zirconium, 

Thorium, 

Cerium, 

Lanthanium, 


Bromine, 
Iodine, 
Selenium, 
Fluorine, 


Didynium, 

Tantalum^ 

Erbium, 

Thallium, 

Ruthenium, 

Rubidium, 

Niobium, 

Indium. 


Boron, 
Silicon, 
Tellurium. 


Of  the  elementary  substances  now  enumerated,  about  fourteen  constitute 
the  great  mass  of  our  earth  and  its  atmosphere.  The  remainder  occur  only 
in  comparatively  small  quantities,  while  nearly  a  third  of  the  whole  number 
is  so  rare  that  their  uses  in  the  great  economy  of  nature  are  not  understood, 
nor  have  they  as  yet  admitted  of  any  useful  application. 

The  science  of  Geology  reveals  to  us  the  fact  that  granite  appears  to  be 
the  foundation  of  the  crust  of  the  earth ;  and  in  the  granite,  either  in  its 
original  formation  or  in  veins  or  seams  which,  have  been  thrown  up  by 
subterranean  forces  into  the  granite,  all  of  the  elementary  substances  which 
have  been  enumerated  are  to  be  found.  A  chart  is  presented  below  in 
which  the  materials  composing  the  strata  of  the  crust  of  the  earth  are 
enumerated,  together  with  a  tabular  view  of  the  composition  of  these 
materials.  It  is  not  contended  that  this  chart  is  perfectly  accurate  in  all 
its  details  ;  but  as  it  affords  an  interesting  and  extensive  subject  of  inves- 
tigation, and  as  it  is  not  to  be  found  elsewhere  in  print,  it  is  thought  that  it 
will  be  well  worth  the  space  which  it  occupies,  although  a  rigid  classifies* 
tion  would  exclude  it  from  this  work. 


OF   MATTER   AND   ITS   PROPERTIES.  21 

Dr.  Boyntoris  Chart  of  Materials  that  enter  into  the  Composition  of  Granite. 


Quartz      .                   . 

1 

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Feldspai   
Albite       

Mica      .    . 

Hornblende     .... 

Chlorite    

Talc  .... 

Hypersthene    .... 
Actrnolite     

Steatite     

Serpentine    

Schorl 

Garnet  

M.  Garnet    .    . 
Clay  . 

Green  Sand  
Carbonate  of  Lime  .    . 
Carbonate  cf  Magnesia 

What  are  31.  There  are  seven  essential  *  properties  be- 
the essen-  iongmg  to  matter,  namely,  1.  Impenetrability; 
crties  of  2.  Extension ;  3.  Figure ;  4.  Divisibility ;  5.  In* 
Matter?  dostructibility ;  6.  Inertia;  7.  Attraction. 

What  is        32.  IMPENETRABILITY.  —  Impenetrability  is  th« 

Impene-  *  .  ,    .  ,.          /. 

trabilityl  power  ot  occupying  a  certain  portion  01  space,  so 

*  An  essential  property  .  of  a  body  is  that  which  is  necessary  to  the 
absolute  existence  of  the  body.  All  matter  in  common  possesses  these 
essential  properties,  and  no  particle  of  matter  can  exist  without  any  ane  of 
them.  Different  bodies  possess  other  different  properties  which  are  not 
essential  to  their  existence,  such  as  color,  weight,  brittleness,  hardness. 
Ac.  These  are  called  accidental  properties,  as  they  depend  ou  circum- 
stances not  essential  to  the  very  existence  of  a  body. 


J2  NATURAL   PHILOSOPHY. 

that   where  one  body  is  another  cannot   be  without   Jig- 
placing  it. 

33.  This  property,  Impenetrability,  belongs  to  all  bodies  and 
forms  of  matter ,  whether  solid,  fluid,  gaseous,  or  vesicular. 

The  impenetrability  of  common  air  may  be  shown  by  immersi  n  g 
an  inverted  tumbler  in  a  vessel  of  water.  The  air  prevents  the 
water  from  rising  into  the  tumbler.  An  empty  bottle,  also,  forcibly 
held  horizontally  under  the  water,  will  exhibit  the  same  property  , 
for  the  bottle,  apparently  empty,  is  tilled  with  air,  which  escapes 
in  bubbles  from  the  bottle  as  the  water  enters  it.  But,  if  the  bottle 
be  inverted,  the  water  cannot  enter  the  bottle,  on  account  of  the 
impenetrability  of  the  air  within.* 

*  This  circumstance  explains  the  reason  why  water,  or  any  other  liquid, 
poured  into  a  tunnel  closely  inserted  in  the  mouth  of  a  decanter,  will  rua 
over  the  sides  of  the  decanter.  The  air  filling  the  decanter,  and  having 
no  means  of  escape,  prevents  the  fluid  from  entering  the  decanter  ;  but,  if 
the  tunne]  be  lifted  from  the  decanter  but  a  little,  so  as  to  afford  the  air  an 
opportunity  to  escape,  the  water  will  then  flow  iutc  the  decanter  in  an  un- 
interrupted stream. 

When  a  nail  is  driven  into  wood  or  any  other  substances,  it  forces  the 
particles  asunder  and  makes  its  way  between  them. 

An  experiment  was  made  at  Florence,  many  years  agor  to  show  the  im- 
penetrability of  water.  A  hollow  globe  of  gold  was  filled  with  water  and 
subjected  to  great  pressure.  The  water,  having  no  other  means  of  escape, 
teas  seen  to  exude  from  the  pores  of  the  gold. 

The  reason  why  fluids  appear  less  impenetrable  than  solids  is  that  th-j 
particles  which  compose  the  fluids  move  easily  among  themselves,  on  account 
of  their  slight  degree  of  cohesion,  and  when  any  pressure  is  exerted  upon  a 
fiuid  the  particles  move  readily  into  the  unoccupied  space  to  which  they 
have  access.  But,  if  the  fluid  be  surrounded  on  all  sides,  and  have  no 
means  of  escape,  it  will  be  found  to  possess  the  property  of  impenetrability 
in  no  less  a  degree  than  solid  bodies. 

A  well-known  fact  seems,  at  first  view,  to  be  at  variance  with  this  state- 
ment. When  a  ve-'sel  is  filled  to  the  brim  with  water  or  other  fluiu,  a  con- 
siderable portion  01  salt  may  be  dropped  into  the  fiuid  without  causing  the 
vessel  to  overflow.  And,  when  salt  has  been  added  until  the  water  can 
hold  no  more  in  solution,  a  considerable  quantity  of  sugar  can  be  added  in 
a  similar  manner.  The  explanation  of  this  familiar 
fact  is  as  follows  The  particles  of  the  sugar  are 
smaller  than  the  particles  of  the  salt,  and  the  particles 
of  the  salt  arc  smaller  than  the  particles  which  compose 
the  water.  Now,  supposing  all  of  these  particles  to  be 
globular,  they  will  arrange  themselves  as  is  represented 
in  Fig.  1,  in  which  the  particles  of  the  water  are  indi- 
cated by  the  largest  circles,  those  of  the  salt  by  the 
pext  in  size,  and  those  of  the  sugar  by  the  smallest. 

Familiar  Experiment.  —  Fill  a  bowl  or  tumbler  with  peas,  then  pour  on 
Uie  peas  mustard-seed  or  fine  grain,  shaking  the  vessel  to  cau?e  it  to  fill  tho 
racar.t  spaces  between  the  peas.  In  like  manner  add,  successively,  fine  sand, 
cater,  salt  and  sugar.  This  will  afford  an  illus  -ration  of  the  apparent  paraduy 
»i  i;vo  bodies  occupying  the  same  space,  and  ;  how  that  it  is  ouJy 


OF   MATTEE   AND   ITS   PROPEETIES.  23 

What  is        34:'  EXTENSION.  —  Extension  is  but  another 

Extern       name  for  bulk  or  size,  and  it  is  expressed  by  the 

terms  length,  breadth  or  width,  height,  depth  and 

thickness. 

NOTE.  -  -  Length  is  the  extent  from  end  to  end.  Breadth  or  width  is  the 
extent  from  side  to  side.  Height,  depth  or  thicKness,  is  the  extent  frona 
the  top  to  the  bottom.  The  measure  of  a  body  frun  the  bottom  to  the  top 
is  f-allei  height  ;  from  the  top  to  the  bottom,  is  called  depth.  Thus  we 
speak  of  the  depth  of  a  well,  the  height  of  a  house,  £c. 

14- iiat  zs        gg    Figure  is  the  form  or  shape  of  a  body. 

36.  Figure  and  Extension  are  separate  properties,  although  both 
may  be  represented  by  the  same  terms,  .length,  breadth,  &c.  But 
they  differ  as  the  words  shape  and  size  differ.  Two  bodies  may  be 
of  the  same  figure  or  shape,  but  of  vastly  different  size.  A  grape  and 
an  orange  resemble  eaph  other  in  shape,  but  differ  widely  in  size. 
The  limits  of  extension  constitute  figure,  but  figure  has  no  other 
connexion  with  extension. 

What  is       37.  DIVISIBILITY. —  Divisibility  is  susceptibility 

Dh'isi-          „  ,    .         , .    . ,    , 

bility?       of  being  divided. 

38.  To  the  divisibility  of  matter  there  is  no  known  limit,  nor 
can  we  conceive  of  anything  so  small    that  it  is  not  made  up  of  two 
halves  or  four  quarters.     It  is  indeed  true  that  our  senses  are  quite 
limited  in  their  operation,  and  that  we  cannot  perceive  or  take 
cognizance,  by  means  of  our  senses,  of  many  objects  of  the  existence 
of  which  we  are  convinced  without   their  immediate  and   direct 
testimony. 

39.  Sir  Isaac  Newton  has  shown  that  the  thickest  part  of  a  soap- 
bubble  does  not  exceed  the  two-millionth  part  of  an  inch. 

40.  The  microscopic  observations  of  Ehrcnberg  have  proved  that 
there  are  many  species  of  little  creatures,  called  Infusoria,  so  small 
tnat  millions  of  them  collected  into  a  single  mass  would  not  exceed 
the  bulk  of  a  grain  of  sand,   and   thousands  of    them  might  swim 
side  by  side  through  the  eye  of  a  small  needle. 

41.  In  the  slate  formations  in  Bohemia  these  little  creatures  aro 
found  in  a  fossil  state,  so  small  that  it  would  require  a  hundred  and 
eighty-seven  millions  of  them  to  weigh  a  single  grain. 

42.  A  single  thread  of  the  spider's  web  has  been  found  to  be 
tornposed  of  six  thousand  filaments. 

43.  A  single  grain  of  gold  may  be  hammered  by  a 'gold-beater 
until  it  will  cover  fifty  square  inches  ;  each  square  inch  may  bo 
divided  into  two  hundred  strips  ;  and  each  strip  into  two  hundred 
rarts.     One  of  these  parts  is  only  OIK  two-millionth  part  of  a  graiw 
:f  gold,  and  yet  it  may  be  seen  with  the  naked  eye 


24  NATURAL  PHILOSOPHY. 

44.  The  particles  which  escape  from  odoriferous  objects  also 
aflbrd  instances  of  extreme  divisibility. 

\Vhat  is 

lnde.  45.  INDESTRUCTIBILITY.  —  By  the  Indestructi- 

siructi-      bility  of  matter  is  meant  that  it  cannot  be  destroyed. 

46.  A  body  may  be  indefinitely  divided  or  altered  in  its  form, 
color,  and  other  unessential  properties,  but  it  can  never  be  destroyed 
by  man.     It  must  continue   to  exist   in  some  form,  with   all   its 
essential  properties,  through  all  its  changes  of  external  appearance. 
HE  alone  "  who  can  create  can  destroy."' 

47.  When  water  disappears,  either  by  boiling  ove*  a  fire  or  by 
evaporation   under   the  heat  of  the  sun,  it  is  not  destroyed,  but 
merely  changed  from  a  liquid  to  a  fluid  form,  and  becomes  steam  or 
vapor.     Some  of  its  unessential  properties  are  altered,  but  its  essential 
properties  remain  the  same,  under  all  the  changes  which  it  under- 
goes.    In  the  form  of  water  it  has  no  elasticity  *  and  but  a  limited 
degree  of  compr-*«^ bility.*     But  when    "it  dries   up"    (as  it  is 
called)  it  rises  in  the  form  of  steam  or  vapor,  and  expands  to  such  a 
degree  as  to  become  invisible.     It  then  assumes  other  properties, 
not  possessed  before  (sucb  as  elasticity  and  expansibility),  it  ascends 
In  the  air  and  forms  clouds  ;  these  clouds,  affected  by  the  temperature 
of  the  air  and  other  agents,  again  fall  to  the  earth  in  the  form  of 
rain,   hail,  snow  or  sleet,  and  form  springs,  fountains,  rivers,  &c 
The  water  on  or  in  the  earth,  therefore,  is  constantly  changing  its 
shape  or  situation,  but  no  particle  of  it  is  ever  actually  destroyed. 

48.  Substances  used  as  fuel,  whether  in  the  form  of  wood,  coal, 
or  other  materials,  in  like  manner  undergo  many  changes  by  the 
process  of  combustion.     Parts  of  them  rise  in  the  form  of  smoke, 
part  ascends  in  vapor,  while  the  remainder  is  reduced  to  the  form 
of  ashes  ;  but  no  part  is  absolutely  destroyed.     Combustion  merely 
disunites  the  simple  substances  of  which  the  burning  materials  ar« 
composed,  forming  them  into  new  combinations  ;  but  every  part  still 
continues  in  existence,  and  retains  all  the  essential]  properties  of 
bodies. 

What  is  49.  INERTIA. — Inertia  J  is  the  resistance  of 
Inertia  ?  matter  to  a  change  of  state,  whether  of  motion  or 
of  rest. 

*  Late  writers  assert  that  water  has  a  slight  degree  both  of  elasticity  and 
expansibility. 

t  The  reader  wil]  bo  careful  to  carry  in  his  mind  what  is  meant  by  the 
terra  an  essential  property.  It  is  explained  in  the  note  to  No.  31,  page  21 

}  The  lite-rid  meaning  of  inertia  is  inactivity,  and  implies  inability  tc 
change  a  ttat*  of  rest  or  of  motion.  A  clear  and  distinct  understanding  0) 
this  property  of  all  matter  is  essential  in  all  the  departments  cf  material 
philosophy.  All  matter,  mec.v  mically  considered,  must  be  in  a  state  either 


OF   MATTER   AND    US   PROPERTIES.  25 


50  A  body  at  rest  cannot  put  itself  in  motion,  nor  can  a 
in  motion  stop  itself  This  incapacity  to  change  its  state  from  rest 
to  motion,  or  from  motion  to  a  state  of  rest,  is  what  is  implied  by 
the  term  inertia. 

51.  It  follows,  therefore,  from  what  has  Just  been  stated,  that 
when  a  body  is  in  motion  its  inertia  will  cause  it  to  continue  to  ino-ve 
until  its  motion  is  destroyed  by  some  other  force. 

52.  There  are  two  forces  constantly  exerted  around  us  which 
tend  to  destroy  motion,  namely,  gravity  and  the  resistance  of  the  air. 
All  motion   caused  by  animal  or  mechanical  power  is  affected  by 
these  two  forces.      Gravity  (which  will  presently   be   explained) 
eausf*  all  bodies,  whether  in  motion  or  at  rest,  to  tend  towards  the 
ceutn   of  the  earth,  and  the  air  presents  a  resistance  to  all  bodiea 
moving  in  it.     Could  these  and  all  other  direct  Fig.  2. 
obstaJes  to  motion  be  set  aside,  a  body  when 

once  put  in  motion  would  always  remain  in    *\        M.      r*HJ 
motion,  and  a  body  at  rest,  unaffected  by  any  **"*- 
external  force,  would  always  remain  at  rest.* 

53.  Experiment  to  illustrate  Inertia. 
— Fig.  2  represents  the  simple  apparatus 
employed  for  illustrating  the  inertia  of  a 
body.    A  card  is  placed  on  the  top  of  a 

stand,  and  a  ball  is  balanced  on  the  card.    A  quick  motion 

of  motion  o/  rest  ;  and,  in  whatever  state  H  may  be,  it  must  remain  in  that 
state  until  a  change  is  effected  by  some  r  flicient  cause,  independent  of  the 
body  itself.  A  body  placed  upon  another  body  in  motion  partakes  of  the 
motion  of  the  body  on  which  it  is  placid.  But,  if  that  body  be  suddenly 
stopped,  the  superincumbent  body  will  not  stop  at  the  same  time,  unless  it 
be  securely  fastened.  Thus,  if  a  hors"  moving  at  a  rapid  rate  be  suddenly 
stopped,  the  rider  will  be  thrown  fofyard,  on  account  of  this  inertia  of  his 
body,  unless  by  extra  exertion  he  secures  himself  on  the  saddle  by  bracing 
his  feet  on  the  stirrups.  On  the  contrary,  if  the  horse,  from  a  state  of  rest, 
start  suddenly  forward,  the  rider  will  be  thrown  backwards.  For  the  same 
reason,  when  a  person  jumps  from  a  vehfcle  in  motion  to  the  ground,  his 
body,  partaking  of  the  motion  of  the  vehicle,  cannot  be  suddenly  brought 
to  a  state  of  rest  by  his  feet  resting  on  the  ground,  but  will  be  throwr 
forward  in  the  direction  of  the  motion  which  it  has  acquired  from  the 
vehicle.  This  is  the  reason  that  bo  many  accidents  happen  from  leaping 
from  a  vehicle  in  motion. 

*  In  the  absence  of  all  positive  proof  from  the  things  around  us  ol 
the  statement  just  made,  we  may  find  from  the  truths  which  astronomy 
teaches  that  inertia  is  one  of  the  necessary  properties  of  all  matter.  Th* 
heavenly  bodies,  launched  by  the  hand  of  their  Creator  into  the  fields  of, 
infinite  space,  with  no  opposing  forca  but  gravity  alone,  have  performed 
their  stated  revolutions  in  perfect  consistency  with  the  character  which 
'.his  property  gives  them  ;  and  all  the  calculations  which  have  been  made 
with  respect  to  them,  verified  as  they  Lave  repeatedly  been  by  v-  '"-ration, 
havo  been  predicated  on  their  possess;on  of  this  necessary  piv,  «.-»>  of  i»JI 


#6  NATURAL   PHILOSOPHY. 

is  then  given  to  the  card  by  means  of  a  spring,  and  the 
card  flies  off,  leaying  the  ball  on  the  top  of  the  stand.* 

54.  Nature  seems  to  have  engrafted  some  knowledge  of  mechan- 
ical laws  on  the  instinct  of  animals.    When  an  animal,  and  especially 
a  large  animal,  is  in  rapid   motion,  he  cannot  (on  account  of  the 
inertia  of  his  body)  suddenly  stop  his  motion,  or  change  its  direction  ; 
and  the  larger  the  animal  the  more  difficult  does  a  sudden  stoppage 
become.     The  hare  pursued  by  the  hound  often  escapes,  when  the 
dog  is  nearly  upon  him,  by  a  sudden  turn,  or  changing  the  direction 
of  its  flight,  thus  gaining  time  upon  his  pursuer,  whose  inertia  is  not 
so  readily  overcome,  and  who  is  thus  impelled  forward  beyo^V  the 
spot  where  the  hare  turned. 

55.  Children  at  play  are  in  the  same  manner  enabled  "  ti  ^oitge^ 
their  elder  playmates,  and  the  activity  of  a  boy  will  often  enable 
him  to  escape  the  pursuit  of  a  man. 

56.  It  is  the  effect  of  inertia  to  render  us  sensible  to  mention.    A 
person  in  motion  would  be  quite   unconscious  of  that  state,  were  it 
aot  for  the  obstacles  which  have  a  tendency  to  impede  his  progress. 
In   a  boat  on   smooth   water,  motion  is   perceptible  only   by  the 
apparent  change  in  the  position  of  surrounding  objects ;  but,  if  the 
course  of  the  boat  be  interrupted  by  running  aground,  or  striking 
against  a  rock,  the  person  in  the  boat  would  feel  the  shock  caused 
by  the  sudden  change  from  a  state  of  motion  to  a  state  of  rest,  and, 
unless  secured  to  his  seat  in  the  boat,  he  would  be  precipitated 
forward 

What  is  At-  57.  ATTRACTION. — Attraction  is  the  tendency 
traction?  which  different  bodies  or  portions  of  matter 
have  to  approach  or  to  adhere  to  each  other. 

What  is  the  ^'  Every  portion  of  matter  is  attracted  by  everj 
MW  of  At-  other  portion  of  matter,  and  this  attraction  is  the 
ractwn .  gtr0nger  in  proportion  to  the  quantity  and  the  dis- 
tance. The  larger  the  quantity  and  the  less  the  distance,  thw 
stronger  is  the  attraction.! 

*  The  ball  remains  on  the  pillar  in  this  case  not  solely  from  its  tncrtif 
but  because  sufficient  motion  is  not  communicated  to  the  b«ll  by  the  fi  Lo- 
tion of  the  card  to  counteract  the  effect  of  gravity  on  the  ball.  If  ihe 
bail,  therefore,  be  not  accurately  balanced  on  the  card,  the  experiment  vilJ 
not  be  successful,  because  the  card  cannot  move  without  communicating  at 
least  a  portion  of  its  motion  to  the  ball. 

f  [N.  B.  This  subject  will  be  more  fully  treated  under  the  head  of 
7rawfy  See  page  33.] 


OF  MATTER   AND  ITS   PROPERTIES.  27 

7 .  ,       59.  There  are  two  kinds  of  attraction — 
How  many  kinds 

of  Attraction  namely,  the  Attraction  of  Gravitation  and 
are  there  ?  the  Attraction  of  Cohesion.  (See  par.  1388.) 

The  former  belongs  to  all  matter,  whatever  its  form,  th<? 
latter  appears  to  belong  principally  to  solid  bodies. 

What  is  thi  60.  The  Attraction  of  Gravitation  is  the 
n/Gravi^  reciprocal  attraction  of  separate  portions  of 
tation  *  matter. 

What  is  the  ®1.  The  Attraction  of  Cohesion  is  that  whicn 
Attraction  causes  the  particles  of  a  bodv  to  cohere  together. 
of  Cohesion?  r£w  jVo.  31.] 

62.  The  attraction  of  cohesion  appears  to  exist  but  in  &  very 
slight  degree,  if  at  all,  in  liquids  and  fluids. 

Exemplify 

the  two  kinds       63.  The  attraction  of  gravitation  causes  a  body, 

of  Attrac-      when   unsupported,  to  fall   to   the  ground.     The 

tion ;  name-  .        , 

ly,  Gravity    attraction  of  cohesion  holds  together  the  particles 

and  Cohesive  Of  a  bO(jy  an(j  causes  them  to  unite  in  masses.* 

Attraction  f  J 

64.  Having   described    the  essential   properties    of   bodies,   we 
come  now  to  the  consideration  of  other  properties  belonging  respect- 
ively to  different  kinds  of  matter  ;  such  as  Porosity,  Density,  Rarity, 
Compressibility,    Expansibility,  Mobility,    Elasticity,    Brittleness, 
RltxibiKty,  Malleability,  Ductility,  Tenacity. 

65.  It  has  already  been  stated  that  matter  consists  of  minute 
particles  or  atoms,  unixed  by  different  degrees  of  cohesive  attraction. 
These  atoms  are  probably  of  different  shapes  in  different  bodies,  and 
the  different  degrees  of  compactness  with  which  they  unite  give 
rise  to  certain  qualities,  which  differ  greatly  in  different  substances 
These   qualities  or   properties  are  described  under   the   names  of 
Porosity,  Density  and  Rarity,  which  will  presently  be  described. 

*  Besides  those  two  kinds  of  attraction,  there  seem  to  be  other  kinds 
ot  attractive  force,  active  in  vegetation  and  in  animal  life,  known  by  the 
names  of  Endosmose  and  Exosmose,  terms  applied  to  the  transmission  of 
gaseous  matter  or  vapors  through  membranous  substances.  See  note  to 
Capillaiy  Attraction,  under  the  head  of  Hydrostatics,  on  page  112. 

Other  kinds  of  attraction,  called  Electrical  and  Magnetical  Attraction, 
will  hereafter  be  considered  under  their  appropriate  head.  The  subject  of 
Chemical  Attraction  or  Affinity  belongs  distinctly  to  the  subject  of  Chemistry 
•ind  will  not,  therefore,  be  cuusidei  od  in  this  work 


'28  NATURAL    PHILOSOPHY. 

66.  Besides  the  property  of  attraction  possessed  by  une  particles 
or  atoms  of  which  a  body  is  composed,  there  seems  to  be  another 
property,  of  a  nature  directly  opposite  to  attraction,  which  exerts 
.tself  with  a  repulsive  force,  to  prevent  a  closer  approximation  of 
the  particles  than  that  which  by  the  law  of  their  nature  they  assume. 
This  property  is  called  repulsion.  This  repulsion  prevents  the  par- 
ticles or  atoms  from  coming  into  perfect  contact,  so  that  there  must 
he  small  spaces  between  them,  where  they  do  not  absolutely  touch 
one  another.  [See  Figure  1st.]  These  spaces  are  called  pores,  and 
where  they  exist  give  rise  to  that  prop^rtj-  or  quality  described 
under  the  name  of  Porosity. 

What  is  M-  POROSITY. — Porosity  implies,  therefore,  that 

Porosity?  there  are  spaces,  or  pores,  between  the  particles  or 
atoms  which  form  the  mass  of  a  body. 

68.  DENSITY. — "When  the  pores  are  few,  so  that  a  large  number 
of  particles  unite  in  a  small  mass,  the  body  is  called  a  dense  body. 

What  is  69.  Density,  therefore,  implies  the  closeness  or 

Density?  compactness  of  the  particles  which  compose  any 
substance. 

70.  RARITY.  —  When  the  pores  in  any  substance  are  numerous, 
so  that  the  particles  which  form  it  touch  one  another  hi  only  a  few 
points,  the  body  is  called  a  rare  body. 

What  is  71.  Rarity,  therefore,  is  the  reverse  of  density, 
Rarity  1  an(j  implies  extension  of  bulk  without  increase  in 
the  quantity  of  matter. 

72.  From  what  has  now  been  stated  it  appears  [See  No.  67]  that 
the  particles  of  a  body  are  connected  together  by  a  system  of  attrac- 
tions and  repulsions  which  give  rise  to  distinctions  which    have 
already  been  described.    It  remains  to  be  stated  that  these  attractions 
and  repulsions  differ  much  in  degree  in  different  substances,  and  this 
difference  gives  rise  to  other  properties,  which  will  now  be  explained, 
under  their  appropriate  names. 

73.  COMPRESSIBILITY. — When  the  repulsion  of  the  particles  of  any 
lubstance  can  be  overcome  and  the  mass  can  be  reduced  within 
narrower  limits  of  extension,  it  is  said  to  possess  the  property  of 
Compressibility  * 

*  Compressibility  differs  from  Contractibility  rather  in  cause  than  hi 
effect.  Contractibility  implies  a  change  of  bulk  caused  by  change  of 
i»mperature,  or  any  other  agency  not  mechanical.  Compressibility  implies 
Jlmt  the  diminution  of  bull"  is  caused  by  some  external  mechanical  force 


OF   MATTER   AND    ITS    IMtOPERTlES.  2^ 

What  «         ^'  Compressibility,  therefore,  may  be  defined,  tha 
Comprey  susceptibility   of  a   reduction    of   the   limits  of    ex- 


75.  This  property  is  possessed  by  all   known  substances,  Nut  in 
very  different  degrees,  —  some  substances  requiring  but  little  force 
to  compress  them,  others  resisting  very  great  forces  ;  but  it  is  not 
known  that  there  is  any  substance  unsusceptible  of  compression  .  if 
a  sufficient  force  be  applied  * 

76.  Liquids  in  general  are   less  easily  compressed   than   solid 
bodies  ;  so  much  so,  indeed,  that  in  practical  science  they  are  gen- 
erally considered   as   incompressible.     Under   a  very  considerable 
mechanical  force,  a  siight  degree  of  compression  has  been  observed.  \ 

77.  EXPANSIBILITY.  —  The  system  of  attractions  and  repulsions 
among  the  particles  of  a  body  are  sometimes  so  equally  balanced 
that  they  exist,  as  it  were,  in  an  equilibrium.     In  other  cases  the 
repulsive  energy  is  so  great  as  to  predominate  when  the  attractive 
force  is  unaided.     When  the  repulsive  energy  is  permitted  to  act 
without  restraint,  it  forces  the  particles  asunder  and  increases  the 
limits   of    extension,    giving  rise   to   another   property  of  matter 
possessed  by  many  bodies,  but  in  an  eminent  degree  by  matter  in  a 
gaseous  form.     This  property  is  called  Expansibility. 

What  is        78.  Expansibility,  t  therefore,   may  be  defined 
Expansi-  as  that  property  of  matter  by  which  it  is  enabled 
1  ^         to  increase  its  limits  of  extension. 

79.  ELASTICITY.  —  When  the  atoms  or  particles  which  constitute 
a  body  are  so  balanced  by  a  system  of  attractions  and  repulsions 
thf.t  they  resist  any  force  which  tends  to  change  the  figure  of  the 

*  Sir  Isaac  Newton  conjectured  that  if  the  earth  were  so  compressed 
as  to  be  absolutely  without  pores,  its  dimensions  might  not  exceed  a  cubio 
inch. 

f  Under  a  pressure  of  fifteen  pounds  on  a  square  inch,  water  has  been 
diminished  in  bulk  only  by  about  forty-nine  parts  in  a  million.  Under  a 
pressure  of  fifteen  thousand  pounds  on  a  square  inch,  it  was  compreased 
by  about  one-twentieth  of  its  volume.  The  experiment  was  tried  in  a  cannon, 
and  the  cannon  was  burst. 

^  Expansibility  and  Dilatability  are  but  different  names  for  the  same 
property  ;  but  expansion  implies  an  augmentation  of  the  bulk  or  volume, 
dependent  on  mecnanical  agency,  while  dilatation  expresses  the  same 
condition  produced  by  some  physical  cause  not  properly  falling  under  the 
denomination  of  mechanical  force.  Thus  heat  dilates  most  substances, 
while  cold  contracts  them.  It  is  on  this  principle  that  the  thermometer  is 
constructed.  [See  page  149,  No.  546.] 

All  gaseous  bodies  are  invested  with  the  property  of  dilatability  to  an 
unlimited  degree,  by  means  of  which,  when  unrestrained,  they  will  expand 
spontaneously,  and  that  without  the  application  of  any  external  agency  *<* 
a  degree  to  which  there  is  no  known  limit. 


.50  NATURAL    PHILOSOPHY. 

Dody,  they  will  possess  another  property,  known  by  the   name  ol 
Elasticity. 

What  is        80.   Elasticity,  therefore  is  the  property  which 
Elastic-     causes  a  body  to  resume  its  shape  after  it  has  been 
compressed  or  expanded.* 

81.  Thus,  when  a  bow  or  a  steel  spring  is  bent,  its  elasticity 
causes  it  to  resume  its  shape. 

82.  India  rubber  (or  caoutchouc)  possesses  the  property  of 
elasticity  in  a  remarkable  degree,  but  steam  and  other  bodies  in 
a  gaseous  form  in  a  still  greater.! 

83.  Ivory  is  endowed  with  the  property  of  elasticity  in  a  remark 
able  degree,  but  exhibits  it  not  so  much  by  the  mere  force  of  pressure, 
out  it  requires  the  force  of  impact  to  produce  change  of  form.  J 

What  is        84.  BRITTLENESS.  —  Brittleness  implies  aptness 

Brittle-  ,        , 

ness?        to  break. 

*  This  property  is  possessed,  in  at  least  some  small  degree,  by  all  sub- 
stances ;  or,  at  least,  it  cannot  be  said  that  any  substance  is  wholly 
destitute  of  elasticity.  Even  water  and  other  liquids,  which  yield  with 
difficulty  to  compression,  recover  their  volume  with  a  force  apparently 
equal  to  the  compressing  force.  But,  for  most  practical  purposes,  many 
substances,  such  as  putty,  wet  paste,  moist  paper,  clay,  and  similar  bodies, 
afford  examples  of  substances  possessing  the  property  of  elasticity  in  so 
slight  a  degree  that  they  are  treated  as  non-elastic  bodies. 

t  The  gases  or  aeriform  bodies  afford  the  most  remarkable  instances 
of  elasticity.  When  water  is  converted  into  steam  it  occupies  a  space 
seventeen  hundred  times  greater  than  the  water  from  which  it  is  formed,  and 
its  elasticity  causes  it  to  expand  to  still  larger  dimensions  on  the  application 
of  heat.  It  is  this  peculiar  property  of  steam,  modified,  as  will  be  explained 
in  a  future  part  of  this  work,  which  is  the  foundation  of  its  application  in 
the  movement  of  machinery.  All  gaseous  bodies  are  equally  elastic. 

^  The  metals  which  are  best  adapted  to  produce  sound  are  those 
which  are  most  highly  elastic.  It  sometimes  happens  that  two  metals, 
neither  of  which  have  any  great  degree  of  hardness  or  elasticity,  when 
combined  in  certain  proportions,  will  acquire  both  of  these  properties. 
Thus  tin  and  copper,  neither  of  which  in  a  pure  state  is  hard  or  elastic, 
when  mixed  in  a  certain  proportion,  produce  a  compound  so  hard  and 
elastic  that  it  is  eminent  for  its  sonorous  property,  and  is  used  for  making 
bells,  Ac 

§  Brittleness    and    hardness   are    properties   which    frequently    accom- 
pany each  other,  and  brittleness  is  not  inconsistent  with  elasticity.     Thu 
glass,  for  instance,  which  is  the  most  brittle  of  all  known  substances,  i 
hignly  elastic.     The  same  body  may  acquire  or  be  divested  of  its  brittle- 
aeA3  according  to  the  treatment  which  it  receives.    Thus  iron,  and  somo 
Dther  metals,  when  hezteJ  and  suddenly  plunged  into  eold  water,  become 
brittle;  but  i?J  in  a  tested  state,  they  are  burled  in  hot  sand,  and  thus  o« 


01'    MATTER    AND    ITS    PROPERTIES.  31 

What  is        85.  FLEXIBILITY.  —  Flexibility  implies  a  dis- 
position  to  yield  without  breaking  when  bent. 


What  is  86.  MALLEABILITY.  —  Malleability  implies  thai 
property  by  means  of  which  a  body  may  be  ro- 
duceci  to  the  form  of  thin  plates  by  means  of  the 

kammer  or  the  pressure  of  rollers 

87.  This  property  is  possessed  in  an  eminent  degree  by  some  of 
the  metals,  especially  gold,  silver,  iron  and  copper,  and  it  is  of  vast 
importance  in  the  arts.     A  knowledge  of  the  uses  of  iron,  and  of  its 
nalleability,  is  one  of  the  first  steps  from  a  savage  to  a  civilized  state 
jf  life. 

88.  The  most  malleable  of  the  metals  is  gold,  which  may  be 
hammered  to  such  a  degree  of  thinness  as  to  require  three  hundred 
and  sixty  thousand  leaves  to  equal  an  inch  in  thickness.* 

89.  DUCTILITY.  —  Some  substances  admit  of  being  extended  simul- 
taneously both  in  length  and  breadth.     Others  can  bo  extended  to 
a  greater  degree  in  length  alone  ;  and  this   property   gives  rise  to 
another  name,  called  Ductility. 

What  is  90.  DUCTILITY.  —  Ductility  is  that  property 
Ductility?  whicn  renders  a  substance  susceptible  of  being 
Irawn  out  into  wire. 

91.  The  same  metals  are  not  always  both  ductile  and  malleable 
to  the  same  degree.     Thus  iron  may  be  beaten  into  any  form,  when 
heated,  but  not  into  very  thin   plates,  but  it  can   be   drawn  into 
extremely  fine  wire.     Tin  and  lead,  on  the  contrary,  cannot  be  drawn 
out  into  small  wire,  but  they  are  susceptible  of  being  beaten  into 
ixtremoly  thin  leaves. 

92.  Gold  and  platinum  have  a  high  degree  both  of  ductility  and 
malleability.     Gold  can  be  beaten  (as  has  already  been  stated)  into 

permitted  to  cool  very  gradually,  they  will  lose  their  brittleness  art£ 
icquire  the  opposite  quality  of  flexibility.  This  process  in  the  arts  u 
(ailed  annealing, 

*  The  malleability  of  the  metals  varies  according  to  their  temper- 
iture.  Iron  is  most  malleable  at  a  white  heat.  Zinc  becomes  malleable  at 
she  temperature  of  300J  or  400°.  Some  of  the  metals,  and  especially  anti- 
mony, arsenic,  bismuth  and  cobalt,  possess  scarcely  any  degree  of  this 
property. 

The  familiar  process  of  welding  is  dependent  on  malleability  The  two 
pieces  of  metal  to  be  welded  are  first  heated  to  that  temperature  at  which 
;hey  are  most  malleable,  and,  the  ends  being  placed  together,  the  particles 
ire  driven  into  such  intimate  connexicn  by  the  welding-tummer  th-it  tlujy 
tubere  Different  metals  may  in  some  easss  be  thus  weldeu  tugetho- 


32  NATURAL    PHILOSOPHY 

leaves  so  thin  that  it  would  require  many  thousands  01  them  to  equaj 
an  inch  in  thickness.  It  has  also  been  drawn  into  wiie  so  attenn 
ated  that  one  hundred  and  eighty  yards  of  it  would  not  weigh  more 
than  a  single  grain.  An  ounce  of  such  wire  would  t>e  more  thai 
fifty  miles  in  length.  But  platinum  can  be  drawn  oven  to  a  fine 
wire  than  this. 

What  is  93.  TENACITY.  —  Tenacity  implies,  the  cohesion 
Tenacity ?0f  the  particles  of  a  body. 

94.  Tenacity  is  one  of  the  great  elements  of  strength.  It  is  th* 
absence  of  tenacity  which  constitutes  brittleness.  Both  implj 
strength,  but  in  different  forms.  Thus  glass,  the  moct  brittle  of  aL 
substances,  has  a  great  degree  of  tenacity.  A  slender  r'/d  of  glass, 
which  cannot  resist  the  slightest  lateral  pressure,  if  suspended 
vertically  by  one  end  will  sustain  a  very  considerable  weight  at  the 
other  end.* 

*  A  knowledge  of  the  tenacity  of  different  substances  ic  of  great  use  in 

the  arts.     The  tenacity  of  metals  and  other  substances  has  oeen  ascertained 

oy  suspending  weights  from  wires  of  the  metals,  or  rc*8  and  cords  of 

different  materials. 

The  following  table  presents  very  nearly  the  weights  sustained  by  wires 

of  different  metals,  each  having  the  diameter  of  aDout  one-twelfth  of  an 

Inch 

Lead,  27  pounds.  Silver,  187  pounds. 

Tin,  34      "  Platinum,  274       " 

Zinc,  109      "  Copper,  302       " 

Gold,  150      "  Iron,  549       " 

Cords  of  different  materials,  but  of  the  same  diametek,  sustained  the  foi 

lowing  weights  : 

Common  flax,  1175  pounds.         New  Zealand  flax,     2380  pounds 

Hemp,  1633       «  Silk,  3400       " 

The  following  table  presents  a  more  extended  list  of  materials.     The 

area  of  a  transverse  section  of  the  rods  on  which  the  experiment  was  tried 

iraa  one  square  inch. 

Pounds  Avoirdupois.  Pounds  Avoirdupois 

English  Oak,  8,600  to  12,000         Tin,  .    7,129 

Fir,  11,000         Lead,  3,146 

Beech,  11,500         Rope,    1    inch   in    circum 

Mahogany,  8,000  ference,  1,000 

Teak,  15,000        Whale    line,    2   inches    in 

Cast  Steel,  134,256  circumference,    spun   by 

Iron  Wire,  93,964  hand,  2,240 

Swedish  Bar-iron,  72,064         Do.,  by  machinery,  3,520 

*       Cast-iron,  18,655         Rope,  3  inches  in  sircum- 

Wrought  Copper,  33,792  ference,  6,628 

Platinum  Wire,  62,987         Do.,  4  inches,  9/J88 

•Silver  Wire,  38,257         Cable,  144  inches,  89,60C 

ttold,  30,888         Do.,  23  inches,  'Z55.36C 

/inc,  22,551 

A  more  particular  aeooui  t  cJ  the  tenacity  of  various  substances  will  b< 


OF    GKAV1TY.  '-*3 

95.  The  tenacity  of  metals  is  much  increased  by  uniting  then?. 
A  compound  consisting  of  five  parts  of  gold  and  one  of  copper  has  a 
tenacity  of  more  than  double  that  of  the  gold  or  copper  alone  ;  and 
brass,  which  is  composed  of  copper  and  zinc,  has  a  tenacity  more 
than  double  that  of  the  copper,  and  nearly  twenty  times  as  great  as 
that  of  the  zinc  alone.  A  mixture  of  three  parts  of  tin  and  one  of 
lead  has  a  tenacity  more  than  double  that  of  the  tin  ;  and  a  mixture 
of  eight  parts  of  lead  and  one  of  zinc  has  a  tenacity  nearly  double 
that  of  the  zinc,  and  nearly  five  times  that  of  the  lead  alone.* 

96  GRAVITY.  —  It  has  already  been  stated  that  matter  in  all  its 
forms,  whether  sclid,  fluid  or  gaseous,  possesses  the  property  of 
attraction.  This  property,  with  its  laws,  is  now  to  be  particularly 
considered,  under  the  name  of  Gravity. 

What  is        97.   Gravity  is  the  reciprocal  attraction  of  sej, 
Gravity?  arate  portions  of  matter. 


A11  bo(iies  attract  each  other  with  a  force  pro- 
bodics  at-     portionate  to  their  size,  density  and  distance  from 
eachother-     [See  No.  59.] 


98.  This  law  explains  the  reason  why  a  body  which  is  not  sup 
ported  falls  to  the  earth.  Two  bodies  existing  in  any  portion  of 
space  mutually  attract  each  other,  and  would  rush  together  were 
they  not  prevented  by  some  superior  force.  Let  us  suppose,  for 
instance,  that  two  balls  made  of  the  same  materials,  but  one  weigh- 
ing 11  pounds  and  the  other  weighing  only  one  pound,  were  ten 
feet  apart,  but  both  were  a  hundred  feet  above  the  surface  of  the 
earth.  According  to  this  law,  the  two  balls  would  rush  together. 
the  lighter  ball  passing  over  nine  feet  of  the  distance,  and  the 
heavier  ball  over  one  foot  ;  and  this  they  would  do,  were  they  not 
both  prevented  by  a  superior  force.  That  superior  force  is  the  earth, 
which,  being  a  much  larger  body,  attracts  them  both  with  a  superior  force. 
This  superior  force  they  will  both  obey,  and  both  will  therefore  fall 
to  the  earth.  As  the  attraction  of  the  earth  and  of  the  balls  is 
mutual,  the  earth  will  also  move  towards  the  balls  while  the  balls 
are  falling  to  the  earth  ;  but  the  size  of  the  earth  is  so  much  greater 
than  that  of  the  balls,  that  the  distance  that  the  earth  would  move 
towards  the  balls  would  be  too  small  to  be  appreciated.  f 


found  in  Barlow's  Essaj  on  the  Strength  of  Timber,  Rennie's  Treatis* 
(in  Phil.  Trans.  1818),  Tredgold's  Principles  of  Carpentry,  and  the  4th  vol. 
oi' Manchester  Memoirs,  by  Mr.  Hodgkinson. 

*  There  are  many  other  specific  properties  of  bodies  besides  those  thai 
have  now  been  enumerate  I,  the  consideration  of  which  belongs  to  th* 
eiieuce  of  Chemistry. 

|  The  earth  is  one  quatrillijn,  that  is.  one  thousand  million  miJiiom 
times  larger  than  the  largest  body  which  had  ever  been  known  to  t'al 


84  NATURAL   PHILOSOPHY. 

99.  The  attraction  of  the  earth  Is  the  cause  of  what  we  call 
weight.  When  we  say  that  a  body  weighs  an  ounce,  a  pound,  or  a 
ton,  we  express  by  these  terms  the  degree  of  attraction  by  which  it 
is  drawn  toward^  the  earth.  Therefore, 

What  is          100.    Weight  is  the  measure  of  the  earth's 
Weight?     attraction* 

101.  As  this  attraction  depends  upon  the  quantity  of  matter 
which  a  body  contains/it  follows  that 

What  bodies  Those  bodies  will  have  the  greatest  weight 

have  the  greatest  which  contain  the  greatest  quantity  of  mat- 
"*»"  ter.f 

102.  TERRESTRIAL  GRAVITY. — It  has  already  been  stated  [see 
No.  97]  that  the  attraction  which  one  mass  of  matter  has  for  another 
is  in  proportion  to  the  quantity  and  the  distance ;  and  that  the 
larger  the  quantity  of  matter  and  the  less  its  distance,  the  stronger 
will  be  the  attraction.     The  law  of  this  attraction  may  be  stated  as 
follows : 

}Vhat  is  1^'   ^vei7  portion  of  matter  attracts  every 

the  law  of      other  portion  of  matter  with  a  force  propor- 
attraction?     , .        ,    ,.       .,  ,,  ...  ,   .  , 

tioned  directly  to  the  quantity,  and  inversely 

as  the  square  of  the  distance. 


through  our  atmosphere.  Supposing,  then,  tnat  such  a  body  should  fall 
through  a  distance  of  one  thousand  feet,  the  earth  would  rise  no  more  than 
the  hundred  billionth  part  of  an  inch,  a  distance  altogether  imperceptible 
to  our  senses. 

The  principle  of  mutual  attraction  is  not  confined  to  the  earth.  It  ex- 
tends to  the  sun,  the  planets,  comets  and  stars.  The  earth  attracts  each 
of  them,  and  each  of  them  attracts  the  earth,  and  these  mutual  attractions 
aie  so  nicely  balanced  by  the  power  of  God  as  to  cause  the  regular  motions 
of  all  the  heavenly  bodies,  the  diversity  of  the  seasons,  the  succession  of 
day  and  night,  summer  and  winter,  and  all  the  grand  operations  which  are 
described  in  astronomy. 

*  When  we  say  that  a  body  weighs  an  ounce,  a  pound,  or  a  hundroJ 
pounds,  we  express,  by  these  terms,  the  degree  of  attraction  by  which  it  i* 
drawn  towards  the  earth. 

f  The  weight  of  a  body  is  not  dependent  solely  on  its  size  or  bulk  ;  its 
density  must  also  be  considered.  If  we  take  an  equal  quantity,  by  measure, 
of  two  substances,  —  lead  and  cork,  for  instance,  —  we  shall  find  that,  although 
both  are  of  the  same  size,  the  lead  will  weigh  much  more  than  the  cork. 
The  Cork  is  more  porous  than  the  lead,  and,  consequently,  the  panicles  of 
<vhioh  it  is  composed  must  be  further  apart,  and  therefore  there  must  be 
fewer  of  them  within  a  given  bulk  ;  while,  in  the  lead,  the  pores  are  much 
smaller,  and  the  particles  will,  therefore,  be  crowded  into  a  much  smaller 
•l>ace 


OF    GKAVITY.  35 

104.  Let  us  now  apply  this  law  to  terrestrial  gravity  —  that  is,  to 
tLe   earth's  attraction  ;  and,  for  that  purpose,  let  us  suppose  four 
balls   of  the  same  size  and  density,  to  be  placed  respectively  as  fol 
lows,  namely: 

The  first  at  the  centre  of  the  earth. 

The  second  on  the  surface  of  the  earth. 

The  third  above  the  earth's  surface,  at  twice  the  distance  of  the 
surface  from  the  centre  (that  distance  being  four  thousand  miles) 

The  fourth  to  be  half  way  between  the  surface  and  the  centre. 

To  ascertain  the  attractive  force  of  the  earth  on  each  of  these  balls. 
we  reason  thus  : 

The  first  ball  (at  the  centre}  will  be  surrounded  on  all  sides  by  at 
equal  quantity  of  matter,  and  it  will  remain  at  rest. 

The  second  ball  will  be  attracted  downwards  to  the  centre  by  the 
whole  mass  below  it. 

The  third  ball,  being  at  twice  the  distance  from  the  surface  (gravity 
decreasing  as  the  square  of  the  distance  increases),  will  be  attracted 
by  a  force  equal  to  only  one-fourth  of  that  at  the  surface. 

The  fourth  ball,  being  attracted  downwards  by  that  portion  of  the 
earth  which  is  below  it,  and  upwards  by  that  portion  which  is  above 
it,  will  be  influenced  only  by  the  difference  between  these  two  oppo 
site  attractions  ;  and,  as  the  downward  attraction  is  twice  as  great  as 
the  upward,  the  downward  attraction  will  prevail  with  half  its 
original  force,  the  other  half  being  balanced  by  the  upward  attrac- 
tion. 

105.  As  weight  is  the  measure  of  the  earth's  attraction,  we  may 
represent  this  principle  by  the  weight  of  the  balls,  as  follows  (sup 
posing  the   weight  of  each  ball,  at  the  surface  of  the  earth,  tv  be  one 
pound)  : 

The  first  ball  will  weigh  nothing. 
The  second  will  weigh  one  pound. 
The  third  will  weigh  one-quarter  of  a  pound. 
The  fourth  will  weigh  one-half  of  a  pound. 
The  law  of  terrestrial  gravity,  then,  may  be  stated  as  follows 


What  '  th  ^^'  ^e  f°rce  °f  gravity  *s  greatest  at  the  sur 
law  of  Ter-  face  of  the  earth,  and  it  deer  oases  upwards  as  the 
restrial  square  of  the  distance  from  the  centre  increases, 
and  downwards  simply  as  the  distance  from  the 
centre  decreases. 

According  to  the  principles  just  stated,  a  body  which  at  th«  sur- 
face of  the  earth  weighs  a  pound  at  the  centre  of  the  earth  wiT 
ireigh  nothing. 

1000  miles  from  the  centre  it  will  weigh  i  of  a  pound 
2000     '-'•        !*'«••       "         "  «        £  of  a  pound. 

3000  "      «        "         "  "        |  of  a  pound 

4000     "        "      "       "         "  "        1  pound. 


NATUIUL    PHILOSOPHY. 


8000  miles  from  the  centre  it  will  weigh  4  of  a  pound 

12000  '  .  "  " 

16000              "  "  '  '  r^. 

20000  '        "  "  ' 

24000  *        "  "  ' 

28000  <        tt  M  t 

32000  '        tt  tt  t  64> 

If  the  priniiples  that  have  now  been  stated  have  been  understood, 
the  solution  of  the  following  questions  will  not  be  difficult. 

107.      Questions  jor  Solution. 

[N.  B.  We  use  the  term  weight  in  these  questions  in  its  philosophical 
sense,  as  "  the  measure  of  the  earth's  attraction  at  the  surface."] 

(1.)  Suppose  that  a  body  weighing  800  pounds  could  be  sunk  500 
miles  deep  into  the  earth, — what  would  it  weigh? 

Solution.  500  miles  is  |  of  4000  miles  ;  and,  as  the  distance  from 
the  centre  is  decreased  by  $ ,  its  weight  would  also  be  decreased  in 
the  same  proportion,  and  the  body  would  weigh  700  pounds. 

(2.)  Suppose  a  body  weighing  2  tons  were  sunk  one  mile  below 
the  surface  of  the  earth,  what  would  it  weigh?  Ans.  1.999571 

(3.)  If  a  load  of  coal  weighs  six  tons  at  the  surface  of  the  earth, 
what  would  it  weigh  in  the  mine  from  which  it  was  taken,  sup- 
posing the  mine  were  at  a  perpendicular  distance  of  half  a  mile 
from  the  surface  ?  Ans.  5. 99925  T7. 

(4.)  If  the  fossil  bones  of  an  animal  dug  from  a  depth  of  5228  feet 
from  the  surface,  weigh  four  tons,  what  would  be  their  weight  at 
the  depth  where  they  were  exhumed?  Ans.  ST.  IScwt.  98lb.  + 

(5.)  If  a  cubic  yard  of  lead  weigh  12  tons  at  the  surface  of  the 
earth,  what  would  it  weigh  at  the  distance  of  1000  miles  from  the 
centre?  Ans.  ST. 

(6.)  If  a  body  on  the  surface  of  the  earth  weigh  4  tons,  what  would 
be  its  weight  if  it  were  elevated  a  thousand  miles  above  the  surface  ? 

Solution.  Square  the  two  distances  4000  and  5000,  &c. 

Tons,    cwt      qre.      Ibs. 
Answer.      2       11       0       20. 

(7.)  Which  will  weigh  the  most,  a  body  of  3000  tons  at  the  dis- 
tance of  4  millions  of  miles  from  the  earth,  or  a  body  of  4000  tons  at 
the  distance  of  3  millions  of  miles  ?  Ans.  .003  T7.  and  .00777.  + 

(8.)  How  far  above  the  surface  of  the  earth  must  a  pound  weight 
be  carried  to  make  it  weigh  one  ounce  avoirdupois  ?  Ans.  12000  mi. 

(9.)  If  a  body  weigh  2  tons  when  at  the  distance  of  a  thousand 
miles  above  the  surface  of  the  earth,  whai  vt;  Id  it  weigh  at  the 
surface?  Ans.  3T.  Zcwt.  50Z5. 

(10.)  Suppose  two  balls  ten  thousand  miles  apart  were  to  ap- 
proach each  other  under  the  influence  of  mutual  attraction,  t.h« 
weight  of  one  being  represented  by  15,  that  of  the  other  by  3«j 
dow  far  ^vould  each  move?  Ans.  6666f  mi.  and  $38?»i  «*». 


OP  GRAVITY  37 

(11.)  ^  hich  would  have  the  stronger  attraction  on  ine  eartn,  a  body 
at  the  distance  of  95  millions  of  miles  from  the  earth,  with  a  weight 
represented  by  1000,  or  a  body  at  the  distance  represented  by  95,  and 
a  weight  represented  by  one?  Ans.  As  ^^l^m  to  ^Vs- 

(12.)  Supposing  the  weight  of  a  body  to  be  represented  by  4  ana 
its  distance  at  6,  and  the  weight  of  another  body  to  be  6  and  its 
distance  at  4,  which  would  exert  the  stronger  power  of  attrac- 
tion? Ans.  The  second,  as  £  to  £. 

108.  THE  CENTRE  OF  GRAYITY.  —  As  every  part  of  a  body  possesses 
the  general  property  of  attraction,  it  is  evident  that  the  attractive 
force  of  the  mass  of  a  body  must  be  concentrated  in  some  point  ;  and 
this  point  is  called  the  centre  of  gravity  of  the  body. 

What  is  the  109.  The  Centre  of  Gravity  of  a  body  is  the 
Gravity  of  a  Point  about  which,  all  the  parts  balance  each 
*^y  -?  other. 

110.  This  point  in  all  spherical  bodies  of  uniform  density  will  be 
the  centre  of  sphericity. 

Ill  As  the  earth  is  a  spherical  body,  its  centre  of  gravity  la 
at  the  centre  of  its  sphericity. 

112.  When  bodies  approach  each  other  under  the  effect  of  mutual 
Attraction,  they  tend  mutually  to  approach  the  centre  of  gravity  of 
each  other. 

113.  For  this  reason,  when  any  body  falls  towards  the  earth  its 
motion  will  be  in  a  straight  line    towards  the  centre  of  the  earth 
No  two   bodies  from  different  points  can   approach         Fi    3 

the  centre  of  a  «phere  in  a  parallel  direction,  and  no 
two  bodies  suspended  from  different  points  can  hang 
parallel  to  one  anotherj* 

114.  Even  a  pair  of  scales  hanging  perpendicularly 
to  the  earth,  as   represented  in    Fig.  3,  cannot  be 
exactly  parallel,  because  they  both  point  to  the  same 
spot,  namely,  the  centre  of  the  earth.     But  their 
convergency  is  too  small  to  be  perceptible. 


What  is  a  ^^-  ^ke  Direction  in  which  a  falling  body  ap- 
Vertical  preaches  the  surface  of  the  earth  is-called  a  Vertical 
l'ine?  Line. 

No  two  vertical  lines  can  be  parallel. 

116.  A  weight  suspended  from  any  point  will  always  assume  <i 
vertical  position.* 

*  Carpenters,  masons  and  other  artisans,  make  use  of  a  weight  of  lead 
suspended  at  rest  by  a  string,  for  the  purpose  of  ascertaining  whether  their 
work  stands  in  a  vertical  position.  To  this  implement  they  give  the  n«n>« 
*f  plumb  -line,  from  the  LatUi  woH  jjiwf-nm  ,  lead 


88  NATURAL    PHILOSOPHY. 

117  All  bodies  under  the  influence  of  terrestrial  gravity  will  full 
to  the  surface  of  the  earth  in  the  same  space  of  time,  when  at  an 
equal  distance  from  the  earth,  if  nothing  impede  them.  But  the 
air  presents  by  its  inertia  a  resistance  to  be  overcome.  This  resist- 
ance can  be  more  easily  overcome  by  deLse  bodies,  and  therefore  the 
rapidity  of  the  fall  of  a  body  will  be  in  proportion  to  its  density. 

To  what  is 

a^ofthe         118'  The  resistance  of  the   air  to  toe  fall  of  a 
air  to  a  fall-  body  is  in   direct  proportion  to  the  extent  of  its 

ing  body        surface. 
propor- 
tioned * 

119.  Heavy  bodies  can  be  made  to  float  in  the  air,  instead  of 
falling  immediately  to  the  ground,  by  making  the  extent  of  their 
surface  counterbalance  their  weight.     Thus  gold,  which  is  one  of 
the  heaviest  of  all  substances,  when  spread  out  into  thin  leaf  is  not 
attracted  by  gravity  with  sufficient  force  to  overcome  the  resistance 
of  the  air ;  it  therefore  floats  in  the  air,  or  falls  slowly.     A  sheet 
of  paper  also,  for  the  same  reason,  will  fall  very  slowly  if  spread 
open,  but,  if  folded  into  a  small  compass,  so  as  to  present  but  a  small 
surface  to  the  air,  it  will  fall  much  more  rapidly. 

120.  This  principle  will  explain  the  reason  why  a  person  can 
with  impunity  leap  from  a  greater  height  with  an  expanded  um- 
brella in  his  hand.     The  resistance  of  the  air  to  the  broad  surface 
of  the  umbrella  Checks  the  rapidity  of  the  fall. 

121.  In  the  same  manner  the  aeronaut  safely  descends  from  a 
balloon  at  a  great  height  by  means  of  a  parachute.     But,  if  by  any 
accident  the  parachute  is  not  expanded  as  befalls,  the  rapidity  of  the 
rall  will  not  be  checked.     [See  Fig.  4.] 

122.  EFFECT  OF  GRAVITY  ON  THE  DENSITY  OF  THE  Aia.  —  The  air 
ixtends  to  a  verv  considerable  distance  above  the  surface  of  the  earth.* 
Chat  portion  which  lies  near  the  surface  of  the  earth  has  to  sustain 
she  weight  of  the  portions  above  ;  and  the  pressure  of  the  upper  parts 

*  We  have  no  means  of  ascertaining  the  exact  height  to  which  the  air 
txtends.  Sir  John  Herschel  says  :  "  Laying  out  of  consideration  all  nice 
questions  as  to  the  probable  existence  of  a  definite  limit  to  the  atmosphere, 
beyond  which  there  is,  absolutely  and  rigorously  speaking,  no  air,  it  is  clear 
that,  for  all  practical  purposes,  we  may  speak  of  those  regions  which  are 
more  distant  above  the  earth's  surface  than  the  hundredth  part  of  its 
diameter  as  void  of  air,  and,  of  course,  of  clouds  (which  are  nothing  but 
risible  vapors,  diffused  and  floating  in  the  air,  sustained  by  it,  and  render- 
ing it  tur.Hid,  as  mud  does  water).  It  seems  probable,  from  many  indica 
tions,  that  the  greatest  height  at  which  visible  clouds  ever  exist  does  nui 
exceed  ten  miles,  at  which  height  the  density  of  the  air  is  about  an  eighth 
part  of  what  it  is  at  the  level  of  the  sea."  Although  the  exact  height  to 
whioh  the  atmosphere  extends  has  never  been  ascertained,  it  ceas«ti  tr 
f  eUecf  Ihe  sun's  rajs  at  a  greater  height  than  forty-five  uules 


OF    GRAVITY. 


»f  the  atmosphei-e  on  those  beneath  renders  the  air  near  the  surfaw 
of  the  earth  much  more  dense  than  that  in  the  upper  region*. 

Fig.  4. 


What  effect  123.  The  air  or  atmosphere  exists  in  a  state 
upon  the  °^  compression,  caused  by  Gravity,  which  in- 
air  ?  creases  its  density  near  the  surface  of  the  earth. 

124.  Gravity  causes  bodies  in  a  fluid  or  gaseous  form  to 
move  in  a  direction  seemingly  at  variance  with  its  own  laws. 

Thus  smoke  and  steam  ascend,  and  oil  poured  into  a  vessel  con- 
taining a  heavier  fluid  will  first  sink  and  then  rise  to  the  surface. 
This  seemingly  anomalous  circumstance,  when  rightly  understood 
will  be  found  to  be  in  perfect  obedience  to  the  laws  of  gravi- 
tation. Smoke  and  steam  are  both  substances  less  dense  than 
air,  and  are  therefore  less  -forcibly  attracted  by  gravitation. 
The  air  being  more  strongly  attracted  than  steam  or  smoke,  on 
fccoount  of  its  superior  density,  falls  into  the  space  occupied  by  th« 


NATURAL    PHILOSOPHY. 


fteam,  and  forces  it  upwards.  The  same  reasoning  applies  in  tl>* 
case  of  oil ;  it  is  forced  upwards  by  the  heavier  fluid,  and  both  phb 
nomena  are  thus  seen  to  be  the  necessary  consequences  of  gravity 
The  rising  of  a  cork  or  other  similar  light  .substances  from  the  hot 
torn  of  a  vessel  of  water  is  explained  hi  the  same  way.  This  circum- 
stance leads  to  the  consideration  of  what  is  called  specific  gravity 

What  is  125.  SPECIFIC  GRAVITY.  —  Specific  Gravity 
S^ed/fc^  k  a  term  use(*  *°  exPress  th°  relative  weight  of 
Gravity?  equal  bulks  of  different  bodies.* 

126.  If  we  take  equal  bulks  of  lead,  wood,  cork  and  air,  we  find 
the  lead  to  be  the  heaviest,  then  the  wood,  then  the  cork,  and  lastly 
the  air.     Hence  we  say  that  the  specific  gravity  of  cork   is  greatei 
than  that  of  air,  the  specific  gravity  of  wood  is  greater  than  that  of 
cork,  and  the  specific  gravity  of  lead  greater  than  that  of  wood,  &o. 

127.  From  what  has  now  been  said  with  respect  to  the  attrac 
tion  of  gravitation  and  the  specific  gravity  of  bodies,  it  appears  that, 
although  ihe  earth  attracts  all  substances,  yet  this  very  attraction 
causes  some  bodies  to  rise  and  others  to  fall. 

128.  Those  bodies  or  substances  the   specific  gravity  of  which. 
is  greater  than  that  of  air  will  fall,  and  those  whose  specific  gravity 
is  less  than  that  of  air  will  rise ;  or,  rather,  the  air,  being  more 
strongly  attracted,  will  get  beneath  them,  and.  thus  displacing  them 
will   cause   them    to   rise. 

For  the  same  reason,  cork 
and  other  light  substances 
will  not  sink  in  water,  be- 
cause, the  specific  gravity 
of  water  being  greater,  the 
water  is  more  strongly  at- 
tracted, and  will  be  drawn 
down  beneath  them.  [For 
a  table  of  the  specific 


gravity  of  bodies,  see  Hy- 
drostatics.] 

129.  The  principle  which 
causes  balloons  to  rise  is 
the  same  which  occasions 
the  ascent  of  smoke,  steam, 
&c .  The  materials  of  which 


*  The  quantity  of  matter  in  a  body  is  estimated,  not  by  its  apparent 
size,  but  by  its  weight.  Some  bodies,  as  cork,  feathers,  <fcc.,  are  termed 
light  ;  others,  as  lead,  gold,  mercury,  <fec.,  are  called  heavy.  The  reason 
of  this  is,  that  the  particles  w.hich  compose  the  former  are  not  closely 
packed  together,  and  therefore  they  occupy  considerable  space  ;  while  iu 
the  latter  they  are  joined  more  closely  together,  and  occupy  but  little  room 
A  pound  of  cork  and  a  pound  of  lead,  therefore,  will  dilfer  very  much  in 
apparent  size,  while  they  are  both  equally  attracted  by  the  earth, — that  w 
they  weitfh  the  samo 


MECHANICS.  *1 

&  balloon  is  made,  are  heavier  than  air,  but  their  extension  ia 
greatly  increased,  and  they  are  filled  with  an  elastic  fluid  of  a  dif 
(Went  nature,  specifically  lighter  than  air,  so  that,  on  the  whole,  the 
balloon  when  thus  filled  is  much  lighter  than  a  portion  of  air  of  the 
same  dimensions,  and  it  will  rise. 

130.  Gravity,  therefore,  causes  bodies  which  are  lighter  than 
air  to  ascend,  those  which  are  of  equal  weight  with  air  to  remain 
stationary,  and  those  which  are  heavier  than  air  to  descend.  But 
the  rapidity  of  their  descent  is  affected  by  the  resistance  of  the  air, 
which  resistance  is  proportioned  to  the  extent  of  surface  in  the 
falling  body. 

131.  MECHANICS. —  Mechanics  treats  of  mo- 
\fechanics?   tion'  an(*  the  moving  powers,  their  nature  and 
laws,  with  their  effects  in  machines. 

What  is      -|^2.  Motion  is  a  continued  change  of  place 
Motion  ? 

133.  On  account  of  the  inertia  of  matter,  a  body  j»t  rear,  cannot 
put  itself  in  motion,  nor  can  a  body  in  motion  stop  itself 

What  ,is 

meant  by    134.  That  which  causes  motion  is  called  a  Force. 

a  Force  ? 

135.  That  which  stops  or  impedes  motion  is 


by 
Resist-        called  Resistance.* 

ance  1 

What  things        -j  36    j    reiation  to  motion.  we  must  consider 

are  to  be  con-  ' 

sidered  in  re-    the  force,  the  resistance,  the  time,  the  space 
lotion 

tion  ? 


lotion  to  mo-          Direction,  the  velocity  and  the  momentum 


What  is  the  137.  The  Telocity  is  the  rapidity  with  which 
*fw&teit  a  k°cly  moves ;  and  it  is  always  proportional  to 
proportional  f  the  force  by  which  the  body  is  put  in  motion. 

138.  The  velocity  of  a  moving  body  is  determined  by  the  time 
that  it  occupies  in  passing  through  a  given  space.  The  greater  the 
space  and  the  shorter  the  time,  the  greater  is  the  velocity.  Thus,  if 
one  body  move  at  the  rate  of  six  miles,  and  another  twelve  miles 

*  A  force  is  sometimes  a  resistance,  and  a  resistance  is  sometimes  a  force. 
The  two  terms  are  used  merely  to  denote  opposition.  (See  Appendix,  par.  1387.) 


i  JNATURAL    PHILOSOPHY. 

in  the  same  time,  the  velocity  of  the  latter  is  double  that  of  th« 
former. 

What  is 

the  rule  for       13&.  To  find  the  velocity  of  a  body,  the  space 

«/    passed  over  must  be  divided  by  the  time  employed 

a    moving    in  moving  over  it. 
body? 

Thus,  if  a  body  move  100  miles  in  20  hours,  the  velocity  is  found 
by  dividing  100  by  20.  The  result  is  five  miles  an  hour  * 

140.    Questions  for  Solution. 

(I.)  If  a  body  move  1000  miles  in  20  days,  what  is  its  veljcity  *  Ana. 
60  miles  a  day. 

(2.)  If  a  horse  travel  15  miles  in  an  hour,  what  is  his  velocity  1  A.i» 
t  of  a  mile  in  a  minute. 

(3.)  Suppose  one  man  walk  300  miles  in  10  days,  and  another  200  miles  in 
the  same  time,  —  what  are  their  respective  velocities  T  Ana.  30  «fe  20. 

(4.)  If  a  ball  thrown  from  a  cannon  strike  the  ground  at  the  distance  of 
3  miles  in  3  seconds  from  the  time  of  its  discharge,  what  is  its  velocity  \  A.  I. 

(5.)  Suppose  a  flash  of  lightning  come  from  a  cloud  3  miles  distant  from 
the  earth,  and  the  thunder  be  heard  in  14  seconds  after  the  flash  is  seen; 
how  fast  does  sound  travel  1  Ans.  1181  :|  ft.  per  sec. 

(6.)  The  sun  is  95  millions  of  miles  from  the  earth,  and  it  takes  8J 
minutes  for  the  light  from  the  sun  to  reach  the  earth  ;  with  what  velocity 
does  light  move  *  f  Ans.  191919  +  mi.  per  *«o. 

*  Velocity  is  sometimes  called  absolute,  and  sometimes  relative.  Veloc 
ity  is  called  absolute  when  the  motion  of  a  body  in  space  is  considered 
without  reference  to  that  of  other  bodies.  When,  for  instance,  a  horse  goes 
a  hundred  miles  in  ten  hours,  his  absolute  velocity  is  ten  miles  an  hour. 
Velocity  is  called  relative  when  it  is  compared  with  that  of  another  body. 
Thus,  if  one  horse  travel  only  fifty  miles  in  ten  hours,  and  another  one 
hundred  in  the  same  time,  the  absolute  velocity  of  the  first  horse  is  five 
miles  an  hour,  and  that  of  the  latter  is  ten  miles;  but  their  relative  velocity 
is  as  two  to  one. 

t  .From  the  table  here  subjoined,  the  velocities  of  the  objects  enumerated 
may  be  ascertained  in  miles  per  hour  and  in  feet  per  second,  fractions  ornitt'  d 

TABLE   OP   VELOCITIES. 

Miles  per  hour.  Feet  per  seco««  _ 

A  man  walking 3 4 

A  horse  trotting 7    .    . 10 

Swiftest  race-horse    .        .    .  60 88 

Railroad  train  in  England  .  32    .       , .    .  47 

"  "  America  .18 26 

«  "       .    Belgium  .25 -  ....  36 

«  France       27    .. 40 

"  "  Germany   24 35 

English  steamboats  in         »  , , r,0 

ohaanels S  26 

American  on  the  Hudson  .      18    . .26 

Fast-sailing  vessels       ,    ,      10  .    .       ,                                        ...      14 


MECHANICS.  43 


14L  The  time  employed  by  a  body  in  motion 
ed  by  a  mov-    may  be  ascertained  by  dividing  the  space  by  the 

ing  body  as-     velocity> 
•'ertainedf  J 

Thus,  if  the  space  passed  over  be  100  miles,  and  the  velocity  5  miles 
in  an  hour,  the  time  will  be  100  divided  by  5.  Ans.  20  hours. 

142.     Questions  for  Solution. 

(1.)  If  a  cannon-ball,  with  a  velocity  of  3  miles  in  a  minute,  strike  the 
ground  at  the  distance  of  one  mile,  what  is  the  time  employed  1  Ans.  £  of 
a  minute,  or  20  seconds. 

(2.)  Suppose  light  to  move  at  the  rate  of  192,000  miles  in  a  second  of 
time,  how  long  will  it  take  to  reach  the  earth  from  the  sun,  which  is  95 
millions  of  miles  distant  1  Ans.  8  tnin.  14.07  we.  + 

(3.)  If  a  railroad-car  run  at  the  rate  of  20  miles  an  hour,  how  long  will 
it  take  to  go  from  Washington  to  Boston,—  distance  432  miles  ?  Ana.  21.6  hr. 

(4.)  Suppose  a  ship  sail  at  the  rate  of  6  miles  an  hour,  how  long  will  it 
take  to  go  from  the  United  States  to  Europe,  across  the  Atlantic  Ocean,  a 
distance  of  2800  miles  1  Ans.  19  da.  10  hr.  40  min. 

(5.)  [f  the  earth  go  round  the  sun  in  365  days,  and  the  distance  travelled 
be  540  millions  of  miles,  how  fast  does  it  travel  1  Ans.  1,479,452  s4w  mi. 

(6.)  Suppose  a  carrier-pigeon,  let  loose  at  6  o'clock  in  the  morning  from 
Washington,  reach  New  Orleans  at  6  o'clock  at  night,  a  distance  of  1200 
miles,  how  fast  does  it  fly  1  Aw.  100  mi.  per  hr. 

How  may  the 

*rL,Sy   148-  The  «P«»  Pas8ed  over  may  bo  foi]nd  fey 

in  motion  be    multiplying  the  velocity  by  the  time. 

ascertained  ? 


Miles  per  hour.  Feet }  ft  second 

Slow  rivers 3  ... 4 

Rapid  rivers 7 10 

Moderate  wind 7 1C) 

A  storm   . 36 52 

A  hurricane 80 117 

f'.rnmon  musket-ball    ...       850 1,240 

Rifle-ball 1,000 1,466 

'24  lb.  cannon-ball      ....    1,600 2,346 

Air  rushing  into  a  vacuum  ) 

dt  32^  F 5         W '    •   '    '  1»2% 

\ir-gun    bullet,  air  com- S 

pressed     to    '01    of    its  V       466 „   .   .       .  683 

volume ) 

Sound ,       743  .....            .  1,142 

A  point  on  the  surface  of  >  ,  n«7  ,  ,      ' 

the  earth \    1)037 1'520 

Earth  in  its  orbit  ....      G7,374 98,815. 

The  velocity  of  light  is   186,000  miles  in  a  second  of  time. 
The  veJo:ity  of  the  electric  fluid  is  said   to  be   still  greate»    dad  sow* 
luthonciei  .sti-te  it  to  be  at  tho  rate  of  288  000  miles  in  a  second   *x  time. 
9* 


44  NATURAL   PHILOSOPHY 

Thus,  if  the  velocity  be  5  miles  an  hour,  and  the  time  20  hours 
she  space  will  oe  twenty  multiplied  by  5.     Ans.  100  miles. 

144.  (1.)  If  a  vessel  sail  125  miles  in  a  day  for  ten  days,  how  far  will  it 
&iil  in  that  time  '?  Ans.  1250  mi. 

(2.)  Suppose  the  average  rate  of  steamers  between  New  York  and  Aloan? 
be  about  11  miles  an  hour,  which  they  traverse  in  about  14  hours,  what 
is  the  distance  between  these  two  cities  by  the  river  1  Ans.  154  mi. 

(3.)  Suppose  the  cars  going  over  the  railroad  between  these  two  citie? 
travel  at  the  rate  of  25  miles  an  hour  and  take  8  hours  to  go  over  the  dis- 
tance, how  far  is  it  from  New  York  to  Albany  by  railroad  1  Ans.  200  mi, 

(4.)  If  a  man  walking  from  Boston  at  the  rate  of  2  miles  in  an  hour  reach 
Salem  in  6  hours,  what  is  the  distance  from  Boston  to  Salem  1  Ans.  15  mi. 

(5.)  The  waters  of  a  certain  river,  moving  at  the  rate  of  4  feet  in  a 
second,  reach  the  sea  in  6  days  froin  the  time  of  starting  from  the  source 
of  the  river.  What  is  the  length  of  that  river  1  Ans.  392-jiy  mi. 

(6.)  A  cannon-ball,  moving  at  the  rate  cf  2400  feet  in  a  second  of  time, 
strikes  a  target  in  4  seconds.  What  is  the  distance  of  the  target!  A.  9600  ft 

145.  The  following  formulae  embrace  the  several  ratios  of  the  time,  space 
and  velocity  : 

(1.)  The  space  divided  by  the  time  equals  the  velocity,  or  —  =  v> 

(2.)  The  space  divided  by  the  velocity  equals  the  time,  »r  -  =  t. 
(3.)  The  velocity  multiplied  by  the  time  equals  the  space, 


How  many  _         146.   There  are  three  kinds  of  Motion  — 
are  there  ?         namely,  Uniform,  Accelerated  and  Retarded. 

What  is  147.  When  a  body  moves  over  equal  spaces  in 

Uniform  ,  ..  .,  '.       .        .  ,  ,     _     _   .„ 

Motion?  equal  times,  the  motion  is  said  to  be  uniform. 


What  is  1^'  When  ^ne  spaces  or  distances  over  which 

Accelerated    a  body  moves  in  equal  times  are  successively 
greater,  the  motion  is  said  to  be  Accelerated. 


What  is  -^.  When  the  spaces  for  equal  times  are 

Retarded      successively  less,  the  motion  is  called  Retarded 
Motion?        ,..  ,. 

Motion. 

How  are  Uni-        150.  Uniform  Motion  is  produced  by  the 

form,Acceler-  ,.          „  ,     « 

ated  and  jRe-    momentary  action  of  a  single  force.    Accel- 

tarded  Motion    erated  Motion  is  produced  by  the  continued 

respectively 

produced  ?        action  of  one  or  more  forces.    Retarded  Mo- 

tion is  produced  by  some  resistance. 


MECHANICS.  45 

151.  A  ball  struck  by  a  bat,  or  a  stone  thrown  from  the  hand,  is 
in  theory  an  instance  of  uniform  motion  ;  and  if  the  attraction  of 
gravity  and  the  resistance  of  the  air  could  be  suspended,  it  would 
proceed  onward  in  a  straight  line,  with  a  uniform  motion,  forever. 
But  as  the  resistance  of  the  air  and  gravity  both  tend  to  deflect  it, 
it  in  fact  becomes  first  an  instance  of  retarded,  and  then  of  accel- 
erated motion. 

152.  A  stone,  or  any  other  body,  falling  from  a  height,  is  an 
instance  of  accelerated  motion.     The  force  of  gravity  continues  to 
operate   upon    it   during   the   whole  time  of  its  descent,  and  con- 
stantly increases    its  velocity.  .   It  begins  its  descent  with  the  first 
impulse  of  attraction,  and,  could  the  force  of  gravity  which  gave  it 
the  impulse  be  suspended,  it  would  continue  its  descent  with  a 
uniform  velocity.     But,  while  falling  it  is  every  moment  receiving  a 
new  impulse  from  gravity,  and  its  velocity  is  constantly  increasing 
during  the  whole  time  of  its  descent. 

153.  A  stone  thrown  perpendicularly  upward  is  an  instance  of 
retarded  motion  ;  for,  as  soon  as  it  begins  to  ascend,  gravity  immedi- 
ately attracts  it  downwards,  and  thus  its  velocity  is  diminished.  The 
retarding  force  of  gravity  acts   upon  it   during  every  moment  of  its 
ascent,  decreasing  its  velocity  until  its  upward  motion  is  entirely 
destroyed.     It  then  begins  to  fall  with  a  motion  continually  acceler- 
ated until  it  reaches  the  ground. 

What  time 

does  a  body      ^54.  A  body  projected  upwards  will  occupy  the 

occupy  in  its  .     ..  ,   , 

ascent   and      same  time  m  lts  ascent  an(l  descent. 

descent  ? 

This  is  a  necessary  consequence  of  the  effect  of  gravity,  which 
uniformly  retards  it  in  the  ascent  and  accelerates  it  in  its  descent. 


155;  P«*PKTnAL  MOTION.  -  Perpetual  Mo- 
be  produced?  tion  is  deemed  an  impossibility  in  mechanics, 
because  action  and  reaction  are  always  equal  and  in  con- 
trary directions. 


156'    BJ  the  aCti°H  °f  a  bod7  is  meant  thc 
Reaction  ?         effect  which   it  produces  upon  another  body. 

By  reaction  is  meant  the  effect  which  it  receives  from  the 
body  on  which  it  acts. 

Thus,  when  a  body  in  motion  strikes  another  body,  it  acts  upon  it 
or  produces  motion  ;  but  it  also  meets  with  resistance  from  the  body 
whicb  ip  struck,  and  this  resistance  is  the  reaction  of  the  body. 


46  NATURAL    PHILOSOPHY. 

Uliistratioji  of  Action  and  Reaction  by  tieaiis  of  Elastic  and 
Non-elastu  Balls. 

(1.)  Figure  6  represents  two  ivory  *  balls,  A  and  B, 
of  equal  size,  weight,  &c.,  suspended  by  threads.  If  the 
ball  A  be  drawn  a  little  on  one  side  and  then  let  go, 
it  will  strike  against  the  other  ball  B.  and  drive  it  off  A 
to  a  distance  equal  to  that  through  which  the  first  ball 
fell ;  but  the  motion  of  A  will  be  stopped,  because  when  it  strikes 
B  it  receives  in  return  a  blow  equal  to  that  which  it  gave,  but  in 
a  contrary  direction,  and  its  motion  is  thereby  stopped,  or,  rather, 
given  to  B.  Therefore,  when  a  body  strikes  against  another, 
the  quantity  of  motion  communicated  to  the  second  body  is  lost 
by  the  first ;  but  this  lo«s  proceeds,  not  from  the  blow  given  by 
the  striking  body,  but  from  the  reaction  of  the  body  which  it 
struck. 

(2.)  Fig.  7  represents  six  ivory  balls  of  equal  weight,  suspended 
by  threads.  If  the  ball  A  be  drawn  out,  of  the  perpendicular 
and  let  fall  against  B.  it  will  communicate  its  mo- 
tion to  B,  and  receive  a  reaction  from  it  which  will 
stop  its  own  motion.  But  the  ball  B  cannot  move 
without  moving  0 ;  it  will  therefore  communicate 
the  motion  which  it  received  from  A  to  C.  and 
receive  from  C  a  reaction,  which  will  stop  its  motion. 
In  like  manner  the  motion  and  reaction  are  received  by  each  ol 
the  balls  D,  E,  F ;  but,  as  there  is  n  ball  beyond  F  to  act  upotj 
it,  F  Will  fly  off. 

N.  B.     Thi«  experiment  is  to  bt  performed  -vith  elastic  balls  .  i  ly. 

(3).  Fig.  8  represents  two  tails  of  clay  (which  are  r  ot  elastic* 
of  equal  weight,  suspended  by  s*  rings.     If  the  ball  D 
be  raised  and  let  fall  against  E,  oTily  part  of  the  mo-      F»g.  8. 
tion  of  1)  will  be  destroyed  by  it  (because  the  Dodies 
ai  >  non- elastic),  and  the  two  balls  will  move  on  togeth- 
er to       and  e,  which  are  less  distant  from  the  ver- 
tical line  than  the  ball  D  was  before  H  foil.     Still, 

*  It  will  be  recollected  that  ivory  is  considered  highly  elastic. 


MECHANICS.  47 

however,  action  and  reaction  are  equal,  for  the  action  on  E  ia 
only  enough  to  make  it  move  through  a  smaller  space,  but  sc 
much  of  D's  motion  is  now  also  destroyed. 

157.  It  is  upon  the  principle  of  action  and  reaction  that  birds 
are  enabled  to  fly.     They  strike  the  air  with  their  wings,  and  the 
reaction  of  the  air  enables  them  to  rise,  fall,  or  remain  stationary, 
at  will,  by  increasing  or  diminishing  the  force  of  the  stroke  of  their 
wings.  | 

158.  It  is  likewise  upon  the  same  principle  of  action  and  reaction 
that  fishes  swim,  or,  rather,  make  their  way  through  the  water, 
namely,  by  striking  the  water  with  their  fins.  J 

159.  Boats  are  also  propelled  by  oars  on  the  same  principle,  and 
the  oars  are  lifted  out  of  the  water,  after  every  stroke,  so  as  com 
pletely  to  prevent  any  reaction  in  a  backward  direction. 

^How  may      160.  Motion  may  be  caused  either  by  action  ox 

motion  be  .  ^TT1  i    •,  ..         ...          -MI 

caused?  reaction.  When  caused  by  action  it  is  callecf 
Incident,  and  when  caused  by  reaction  it  is  called  Reflected 
Motion.  § 

*  Figs.  6  and  7,  as  has  been  explained,  show  the  effect  of  action  ancr  re- 
action in  elastic  bodies,  and  Fig.  8  shows  the  same  effect  in  non-elastic  bodies. 
When  the  elasticity  of  a  body  is  imperfect,  an  intermediate  effect  will  bo 
produced  ;  that  is,  the  ball  which  is  struck  will  rise  higher  than  in  case  of 
non-elastic  bodies,  and  less  so  than  in  that  of  perfectly  elastic  bodies;  and 
the  striking  ball  will  be  retarded  more  than  in  the  former  case,  but  not 
stopped  completely,  as  in  the  latter.  They  will,  therefore,  both  move 
onwards  after  the  blow,  but  not  together,  or  to  -the  same  distance  ;  but  in 
this,  as  in  the  preceding  cases,  the  whole  quantity  of  motion  destroyed  in 
the  striking  ball  will  be  equal  to  that  produced  in  the  ball  struck.  Con- 
nected with  "  the  philosophical  apparatus  "  is  a  stand  with  ivory  balls,  to 
give  a  visible  illustration  of  the  effects  of  collision. 

f  The  muscular  power  of  birds  is  much  greater  in  proportion  to  their 
wei-ght  than  that  of  man.  If  a  man  were  furnished  with  wings  sufficiently 
large  to  epable  him  to  fly,  he  would  not  have  sufficient  strength  or  muscular 
power  to  put  them  in  motion. 

J.The  power  possessed  by  fishes,  ot  sinking  or  rising  in  the  water,  ia 
greatly  assisted  by  a  peculiar  apparatus  furnished  them  by  nature,  called 
aii  air-bladder,  by  the  expansion  or  contraction  of  which  they  rise  or  fall, 
an  the  principle  of  specific  gravity. 

§  The  word  incident  implies  falling  upon  or  directed  towards.  The  word 
reflected  implies  turned  ~back.  Incident  motion  is  motion  directed  towards 
any  particular  object  against  which  a  moving  body  strikes.  Reflected  mo- 
tion is  that  which  is  caused  by  the  reaction  of  the  body  which  is  struck. 
Thus,  when  a  ball  is  thrown  against  a  surface,  it  rebounds  or  is  turned 
back.  This  return  of  the  ball  is  called  reflected  motion.  As  reflected  mo- 
tion is  caused  by  reaction,  and  reaction  is  increased  by  elasticity,  it  follows 
that  reflected  motion  is  always  greatest  in  those  bodies  which  are  most  elas- 
tic. For  this  reason,  a  ball  filled  with  air  rebounds  better  than  one  stuffed 
with  bran  or  wool,  because  its  elasticity  is  greater.  For  the  same  reason, 
balls  made  of  caoutchouc,  or  India-rubber,  will  rebound  more  than  those 
which  are  made  of  most  other  substances. 


NATURAL  PHILOSOPHY. 


What,  u  161.  The  angle  *  of  incidence  is  the  angle  formed 
°of  ajlna-  by  the  line  which  the  incident  body  makes  in  its 
detue?  passage  towards  any  object,  with  a  line  perpendic- 
ular to  the  surface  of  the  object. 

*  As  this  book  may  fall  into  the  hands  of  some  who  are  unacquainted  with 
geometrical  figures,  a  few  explanations  are  here  subjoined  : 

1.  An  angle  is  the  opening  made  by  two  lines  which  meet  each  other  in  a 
point.      The  size  of  the  angle  depends  upon  the  opening,  and  not  upon  tht  length 
of  the  lines. 

2.  A  circle  is  a  perfectly  round  figure,  every 
part  of  the  outer  edge  of  which,  called  the  cir- 
cumference, is   equally  distant   from   a   point 
within,  called  the  centre.     [See  Fig.  9.] 

3.  The  straight  lines  drawn  from  the  centre 
to  the  circumference    are   called   radii.     [The 
singular  number  of  this  word  is  radius.}  Thus, 
in  Fig.  »,  the  lines  CD,  C  0,  C  R,  and  C  A,  are 
radii. 

4.  T^e  lines  drawn  through  the  centre,  and 
terminating  in  both  ends  at  the  circumference, 

are  called  diameters.  Thus,  in  the  same  figure,  D  A  is  a  diameter  of  th« 
circle. 

5.  The  circumference  of  all  circles  is  divided  into  360  equal  parts,  called 
degrees.     The  diameter  of  a  circle  divides  the  circumference  into  two  equal 
parts,  of  180  degrees  each. 

6.  All  angles  are  measured  by  the  number  of  degrees  which  they  contain 
Thus,  in  Fig.  9,  the  angle  R  C  A,  as  it  includes  one-quarter  of  the  circle,  is 
an  angle  of  90  degrees,  which  is  a  quarter  of  360.     And  the  angles  R  C  0 
and  0  C  D  are  angles  of  45  degrees. 

7.  Angles  of  90  degrees  are  right  angles  ;   angles  of  less  than  90  degrees, 
acute  angles;  and  angles  of  more  than  90'  degrees  are  called  obtuse  angles, 
Thus,  in  Fig.  9,  RC  A  is  a  right  angle,  0  C  R  an  acute,  and  0  C  A  an  obtuse 
angle. 

8.  A  perpendicular  line  is  a  line  which  makes  an  angle  of  90  degrees  on 
each  side  of  any  other  line  or  surface  ;   therefore,  it  will  incline  neither  to 
the  one  side  nor  to  the  other.     Thus,  in  Fig.  9,  R  C  is  perpendicular  to  D  A. 

9.  The  tangent  of  a  circle  is  a  line  which  touches  the  circumference,  with- 
out cutting  it  when  lengthened  at  either  end.     Thus,  in  Fig.  9,  the  line  RT 
Is  a  tangent. 

10.  A  square  is  a  figure  having  four  equal  sides,  and  four  equal  angles. 
These  will  always  be  right  angles.     [See  Fig.  11.] 

11.  A  parallelogram  is  a  figure  whose  opposite  sides  are  equal  and  parallel 
[See  Figs.  12  and  13.]   A  square  is  also  a  parallelogram. 

12    A  rectangle  is  a  parallelogram  whose  angles  are  right  angles. 

[N.  B.  It  will  be  seen  by  these  definitions  that  both  a  square  and  u 
rectangle  are  parallelograms,  but  all  parallelograms  are  not  rectangles  nor 
equares.  A  square  is  both  a  parallelogram  and  a  rectangle.  Three  thing* 
ure  essential  to  a  square;  namely,  the  four  sides  must  all  be  equal,  they  must 
ttlso  be  parallel,  and  the  angles  must  all  be  right  angles.  Two  things  only 
lire  essential  lo  a  rectangle  ;  namely,  the  angles  must  all  be  right  angles, 
and  the  opposite  sides  must  be  equal  and  parallel.  One  thing  only  is  essen- 
tial to  a  parallelogram;  namely,  the  opposite  sides  must  be  equal  and 


. 
•13    The  diagonal  of  &  square,  of  a  parallelogram,  or  a  rectangle,  \a  e  liu 


MECHANICS. 

Explmn         162.  Thus,   in   Fig.  10,  the  line  Fig.  10. 

Fig.  10     ABC  represents  a  wall,  and  P  B  °  -^ 

a  line  perpendicular  to  its  surface.     O  is  a      _  """"•. 

ball  moving  in  the  direction  of  the  dotted  ^.--** 

line,  0  B.     The  angle  O  B  P  is  the  angle  of  R  -  "" 

incidence. 

What  is  163.  The  angle  or  reflection  is  the  angle  formed 
tfr^fiec-  ty  ^e  PerPendicular  with  the  line  made  by  the 
iion  ?  reflected  body  as  it  leaves  the  surface  against 
which  it  struck. 

Thus,  in  Fig.  10,  the  angle  P  t  R  is  the  angle  of  reflection. 


164.  The  angles  of  incidence  and  re- 

of  incidence  to  the     flection  are  always  equal  to  one  another.* 
angle  of  reflection  ? 

(1.)  Thus,  in  Fig.  10,  the  angle  of  incidence,  0  B  P,  and  the 
angle  of  reflection,  P  B  R,  are  equal  to  one  another  ;  that  is, 
they  contain  an  equal  number  of  degrees. 

What  will  be  the  165  From  what  has  now  been  gtatea  wjth 
course  of  a  body  .  .  in 

in  motion  which  regard  to  the  angles  of  incidence  and  renec- 

strikes  against  t«  it  f0uowg  tnat  when  a  ball  is  thrown 
unothen  fixed  . 

•)ody  ?  perpendicularly  against  an  object  whick 

it  cannot  penetrate,  it  will  return  in  the  same  direction  , 
but,  if  it  be  thrown  obliquely,  it  will  return  obliquely  on 
f,he  opposite  side  of  the  perpendicular.  The  more  06- 
liquely  the  ball  is  thrown,  the  more  obliquely  it  will 
rebound.  \ 

drawn  through  either  of  them,  and  terminating  at  the  opposite  angles.  Thus, 
In  Figs.  11,  12,  and  13,  the  line  A  C  is  the  diagonal  of  the  square,  parallelo- 
gram, or  rectangle. 

*  An  understanding  of  this  law  of  reflected  motion  is  very  import?  nt, 
because  it  is  a  fundamental  law,  not  only  in  Mechanics,  but  also  in  Pyro- 
:iomics,  Acoustics  and  Optics. 

t  It  is  from  a  knowledge  of  these  facts  that  skill  is  acquired  in  many 
different  sorts  of  games,  as  Billiards,  Bagatelle,  <fec.  A  ball  may  also,  on 
tho  aaine  principle,  be  thrown  from  a  gun  against  a  foi  tificatiu'u  so  a*  t* 
roach  a,u  object  out  of  the  range  of  a  direct  shot. 


50  NATURAL    PHILOSOPHY. 

What  is  the  166'  MOMENTUM.— The  Momentum*  of  a 

Momentum  of     body  is  its  quantity  of  motion,  f  and  implies 

an  expression  of  weight  and  velocity  at  the 

same  time. 

How  is  the  The  Momentum  of  a  body  is  ascertained 

Momentum  of  a   ,  ,..  ,   .       ..          ..,,,., 

lody  calculated?  DJ  multiplying  its  weignt  by  its  velocity. 

167.     Thus,  if-the  velocity  of  a  body  be  represented  by  5  and  iU 
eight  by  6,  its  momentum  will  be  30 

How  can  a        1QS.  A  small  or  a  light  body  may  be  made 
ight  body     to  strike  against  another  body  with  a  greater 

be  mule  to     force  than  a  heavier  body  simply  by  giving  it 
do  as  much         ~   .  .    • ..  , '        .       .  ,.         . 

damage  as    sufficien   velocity, —  that  is,  by  making  it  hav« 
a  large  one '  greater  momentum. 

Thus,  a  cork  weighing  \  of  an  ounce,  shot  from  a  pistol  with  the 
velocity  of  100  feet  in  a  second,  will  do  more  damage  than  a  leaden 
shot  weighing  £  of  an  ounce,  thrown  from  the  hand  with  a  velocity 
of  40  feet  in  a  second,  because  the  momentum  of  the  cork  will  b<b 
the  greater. 

The  momentum  of  the  cork  is  4  X  100  =  25. 

That  of  the  leaden  shot  is  j  X  40   =5 

169.    Questions  for  Solution. 

(1.)  What  is  the  momentum  of  a  body  weighing  5  pounds,  moving  with 
ch3  velocity  of  50  feet  in  a  second  1  Ans.  250. 

(2.)  ^'hat  is  the  momentum  of  a  steam-engine,  weighing  3  tons,  moving 
with  the  velocity  of  60  miles  in  an  hour  ]  Ans.  180. 

[N.  B.  It  must  be  recollected  that,  in  comparing  the  momenta  of  bodies 
the  velocities  and  the  time  of  the  bodies  compared  must  be  respectively  of 
the  same  denomination.  If  the  time  of  one  be  minutes  and  of  the  other  be 
t  -jura,  they  must  both  be  considered  in  minutes,  or  both  in  hours.  So. 
with  regard  to  the  spaces  and  the  weights,  if  one  be  feet  all  must  b« 
expressed  in  feet  ;  if  one  be  in  pounds,  all  must  be  in  pounds.  It  is  better, 
however,  to  express  the  weight,  velocities  and  spaces,  by  abstract  numbers 
as  follows  :] 

(3.)  If  a  body  whose  weight  is  expressed  by  9  and  velocity  by  6  is  in 
motion,  what  is  its  momentum  1  An*,  54. 

(4.)  A  body  whose  momentum  is  63  has  a  velocUy  of  9 ;  what  is  its  weight  f 

Ans.  1 

*  The  plural  of  this  word  is  momenta. 

f  The  quantity  of  motion  communicated  to  a  body  does  not  affect  thf 
duration  of  the  motion.  If  but  little  motion  be  communicated,  the  body 
«rill  move  slowly.  If  a  great  degree  be  imparted,  it  will  move  rapidly. 
But  in  both  cases  the  motion  will  continue  until  it  is  destroyed  by  some 
external  force 


N  3.  Tbe  momentum  being  the  product  of  the  weight  and  velocity,  th« 
weight  is  found  by  dividing  the  momentum  by  the  velocity,  and  the  velocity 
J«  found  by  dividing  the  momentum  by  the  weight.] 

(5.)  The  momentum  is  expressed  by  12,  the  weight  by  2  ;  what  is  th« 
velocity  1  Ans.  6. 

(6.)  The  momentum  9,  velocity  9,  what  is  the  weight  7  Ans  1. 

(7.)  -Momentum  36,  weight  6,  required  the  velocity.  Ans.  6. 

(8.)  A  body  with  a  momentum  of  12  strikes  another  with  a  momentum  of 
ti  ;  what  will  be  the  consequence  7  Ans.  Both  have  mom.  of  6. 

[N.  B.  When  two  bodies,  in  opposite  directions,  come  into  collision  ,  they  eac* 
lo^e  an  equal  quantity  of  their  momenta.} 

(9.)  A  body  weighing  15,  with  a  velocity  of  12,  meets  another  coming  in 
the  opposite  direction,  with  a  velocity  of  20,  and  a  weight  of  10  ;  what  wilj 
he  the  effect  7  Ans.  Both  move  *ith  mom.  of  20. 

(10.)  Two  bodies  meet  together  in  opposite  directions  A  has  a  velocity 
of  12  and  a  weight  of  7,  B  has  a  momentum  expressed  by  84.  What,  wib 
^e  the  consequence  7  Ans.  Both  mom.  destroyed. 

(11.)  Suppose  the  weight  of  a  comet  be  represented  by  1  and  it's  velocity 
oy  12,  and  the  weight  of  the  earth  be  expresse'd  by  100  and  its  velocity  bj 
10,  what  would  be  the  consequence  of  a  collision,  supposing  them  to  b« 
tioving  in  opposite  directions  7  Ans.  Both  have  inom.  of  988. 

(12.)  If  a  body  with  a  weight  of  75  and  a  velocity  of  4  run  against  a  mai 
,«tose  weight  is  150,  and  who  is  standing  still,  what  will  be  the  cons« 
quence,  if  the  man  uses  no  effort  but  his  weight?  Ans.  Man  has  vel.  of  !£• 

(13.)  With  what  velocity  must  a  64  pound  cannon-ball  fly  to  be  equally 
effective  with  a  battering-ram  of  12,000  pounds  propelled  with  a  velocity 
of  16  feet  in  a  second  7  Ans.  8000.A 

170.  ATTRACTION  —  LAW  OF  FALLING  BODIES.  —  When   one  bod) 
strikes  another  it  will  cause  an  effect  proportional  to  its  own  weight 
and  velocity  (or,  in  other  words,  its  momentum)  ;  and  the  bod? 
which  receives  the  blow  will  move  on  with  a  uniform  velocity  (if 
the  blow  be  sufficient  to  overcome  its  inertia)  in  the  direction  of 
the  motion  of  the  blow.     But,  when  a  body  moves  by  the  force  of 
a  constant  attraction,  it  will  move   with  a  constantly   accelerated 
motion. 

171.  This  is  especially  the  case  with  falling  bodies.     The  earth 
attracts  them  with  a  force  sufficient  to  bring  them  down  through  & 
certain  number  of  feet  dining  the  first  second  of  time.     While  the 
body  is  thus  in  motion  with  a  velocity,  say  of  sixteen  feet,  the  earth 
still  attracts  it,  aad  during  the  second  second  it  communicates  an 
additional  velocity,  and  every  successive  second  of  time  the  attrac- 
tion of  the  earth  adds  to  the  velocity  in  a  similar  proportion,  so  that 
dniing  any  given  time,  a  falling  body  will  acquire  a  velocity  which, 
in  the  same  time,  would  carry  it  over  twice  the  space  through  which 
it  has  already  fallen.     Hence  we  deduce  the  following  law, 

\\'J,at  ^  the      172.   A  body  falling  from  a  height  will  fall 
ing  bodwt  sixteen  feet  in  the  first  second  of  time,"*  three 


*  This  is  only  an  approximation  to  the  truth  ;  it  actually  falls 
fcnt  and  one  inch  during  the  first  second,  three  times  that  distance  in  th« 
fecoud.  &o      The  questions  proposed  to  be  solved  assume  sixteen  feet  onlv 


52  NATURAL    PHILOSOPHY. 

times   that    distance    in    the    second,    five   times   in   the 
third,  seven  in  the  fourth,  its  velocity  increasing  during 
every  successive  second,  as  the  odd  numbers  1,  3,  5,  7,  9 
H,  13,  &c.* 

The  laws  of  falling  bodies  are  clearly  demonstrated  by  a  mechanical 
arrangement  known  by  the  name  of  "  Attwood^s  Machine,"  in  which  a  small 
weight  is  made  to  communicate  motion  to  two  others  attached  to  a  cord 
passing  over  friction-rollers  (causing  one  to  ascend  and  the  other  to 
descend),  and  marking  the  progress  of  the  descending  weight  by  the  oscil- 
lations of  a  pendulum  on  a  graduated  scale,  attached  to  one  of  the  columns 
of  the  machine.  It  has  not  been  deemed  expedient  to  present  a  cut  of  the 
machine,  because  without  the  machine  itself  the  explanation  of  its  opera- 
tion would  be  unsatisfactory,  with  the  machine  itself  in  view  the  sim- 
plicity of  its  construction  would  render  an  explanation  unnecessary. 

*  The  entire  spaces  through  which  a  body  will  have  fallen  in  any  given 
number  of  seconds  increase  as  the  squares  of  the  times.  This  law  was  dis- 
covered by  Galileo,  and  may  thus  be  explained.  If  a  body  fall  sixteen  feet 
in  one  second,  in  two  seconds  it  will  have  fallen  four  times  as  far,  in  three 
seconds  nine  times  as  far,  in  four  seconds  sixteen  times  as  far,  in  the  fifth 
second  twenty-five  times,  <fcc.,  in  the  sixth  thirty-six  times,  Ac. 

ANALYSIS    OF    THE    MOTION    OP   A    FALLING    BODY. 

Number  of  Seconds.  Spaces.  Velocities.  Total  Space 

1121 
2344 
3569 
4  7  8  16 

6  9  10  25 

6  11  12  36 

7  13  14  49 

8  15  16  64 

9  17  18  81 
10                               19                               20                  .-»-     100 

From  this  statement  it  appears  that  the  spaces  passed  through  by  » 
falling  body,  in  any  number  of  seconds,  increase  as  the  odd  numbers  1,  3, 
6,  7,  9,  11,  &c.  ;  the  velocity  increases  as  the  even  numbers  2,  4,  6,  8,  10, 
12,  &c.  ;  and  the  total  spaces  passed  through  in  any  given  number  of 
seconds  increase  as  the  squares  of  the  numbers  indicating  the  seconds, 
—  thus,  1,  4,  9,  16,  25,  36,  &c. 

Aristotle  maintained  that  the  velocity  of  any  falling  body  is  in  direct 
proportion  to  its  weight;  and  that,  if  two  bodies  of  unequal  weight  were  let 
fall  from  any  height  at  the  same  moment,  the  heavier  body  would  reach  the 
ground  in  a  shorter  time,  in  exact  proportion  as  its  weight  exceeded  that 
of  the  lighter  one.  Hence,  according  to  his  doctrine,  a  body  weighing  two 
pounds  would  fall  in  half  the  time  required  for  the  fall  of  a  body  weighing 
only  one  pound.  This  doctrine  was  embraced  by  all  the  followers  of  that 
distinguished  philosopher,  until  the  time  of  Galileo,  of  Florence,  who  flour- 
ished about  the  middle  of  the  sixteenth  century.  He  maintained  that  the 
velocity  of  a  falling  body  is  not  affected  by  its  weight,  and  challenged  the  • 
adherents  of  the  Aristotelian  doctrine  to  the  test  of  experiment.  The 
leaning  tower  of  Pisa  was  selected  for  the  trial,  and  there  the  experiment 
was  tried  which  rroved  the  truth  of  Galileo's  theory.  A  distinguished 
writer  thus  describes  the  scene  "On  the  appointed  day  tho  dispu*.j»titi 


MECHANICS.  f>^ 

173.  The  height  of  a  building,  or  the  depth  of  a  well,  may  thus 
be  estimated  very  nearly  by  observing  the  length  of  time  wnich  » 
stone  takes  in  /ailing;  from  the  top  to  the  bottom. 

174.  Exercises  far  Solution. 

(1.)  If  a  ball,  dropped  from  the  top  of  a  steeple,  reaches  the  ground  in  5 
seconds,  how  high  is  that  steeple  1 

16-+-48-l-80-f-112-f  144=400  feet ;  or,  5><5=25,  square  of  the  nuinbe* 
of  seconds,  multiplied  by  the  number  of  feet  it  falls  through  in  one  second, 
namely,  16  feet  ;  that  is,  25X16=400  feet. 

(2.)  Suppose  a  ball,  dropped  from  the  spire  of  a  cathedral,  reach  the 
ground  in  9  seconds,  how  high  is  that  spire  1 

16-f-48-|-80-(-l  12+144+176+208+240+272=1296  feet. 

Or,  squaring  the  time  in  seconds,  92=81,  multiplied  by  16=sl2v6.   Ans. 

[It  will  hereafter  be  shown  that  this  law  of  falling  bodies  applies  to  all 
bodies,  whether  falling  perpendicularly  or  obliquely.  Thus,  whether  a 
stone  be  thrown  from  the  top  of  a  building  horizontally  or  dropped  perpen- 
dicularly downwards,  in  both  cases  the  stone  will  reach  the  ground  in  the 
same  time  ;  and  this  rule  applies  equally  to  a  ball  projected  from  a  cannon, 
and  to  a  stone  thrown  from  the  hand.] 

(3  )  If  a  ball,  projected  from  a  cannon  from  the  top  of  a  pyramid,  reach 
the  ground  in  4  seconds,  how  high  is  the  pyramid  1  Ans.  256ft. 

(4.)  How  deep  is  a  well,  into  which  a  stone  being  dropped,  it  reaches  the 
water  6  feet  from  the  bottom  of  the  well  in  2  seconds  1  Ana.  10ft. 

(5.)  The  light  of  a  meteor  bursting  in  the  air  is  seen,  and  in  45  seconds 
a  meteoric  stone  falls  to  the  ground.  Supposing  the  stone  to  have  pro- 
ceeded from  the  explosion  of  the  meteor  perpendicularly,  how  far  from 
the  earth,  in  feet,  was  the  meteor  1  452X16=32,400  feet. 

(6.)  What  is  the  difference  in  the  depth  of  two  wells,  into  one  of  which  a 
stone  being  dropped,  is  heard  to  strike  the  water  in  6  seconds,  and  into 
the  other  in  9  seconds,  supposing  that  the  water  be  of  equal  depth  in  both, 
and  making  no  allowance  for  the  progressive  motion  of  sound  1  A.  896  ft. 

repaired  to  the  tower  of  Pisa,  each  party,  perhaps,  with  equal  confidence. 
It  was  a  crisis  in  the  history  of  human  knowledge.  On  the  one  side  stood 
the  assembled  wisdom  of  the  universities,  revered  for  age  and  science, 
venerable,  dignified,  united  and  commanding.  Around  them  thronged  th« 
multitude,  and  about  them  clustered  the  associations  of  centuries.  On  the 
other  there  stood  an  obscure  young  man  (Galileo),  with  no  retinue  of  fol- 
lowers, without  reputation,  or  influence,  or  station.  But  his  courage  was 
equal  to  the  occasion  ;  confident  in  the  power  of  truth,  his  form  is  erect 
and  his  eye  sparkles  with  excitement.  But  the  hour  of  trial  arrives.  Thf 
balls  to  be  employed  in  the  experiments  are  carefully  weighed  and  scru- 
tinized, to  detect  deception.  The  parties  are  satisfied.  The  one  ball  is 
exactly  twice  the  weight  of  the  other.  The  followers  of  Aristotle  maintaip 
that,  when  the  balls  are  dropped  from  the  tower,  the  heavy  one  will  reach 
the  ground  in  exactly  half  the  time  employed  by  the  lighter  ball.  Galilee 
asserts  that  the  weights  of  the  balls  do  not  affect  their  velocities,  and  that 
the  tunes  of  descent  will  be  equal  ;  and  here  the  disputants  join  issue 
The  balls  are  conveyed  to  the  summit  of  the  lofty  tower.  The  crowd  at^ 
Bemble  round  the  base  ;  the  signal  is  given  ;  the  balls  are  dropped  at  the 
frame  instant  ;  and,  swift  descending,  at  the  same  moment  they  strike  the 
earth.  Again  and  again  the  experiment  is  repeated,  with  uniform  result?; 
Galileo's  triumph  was  complete  ;  not  »  shadow  of  a  doubt  remained  I 
["The  Orbs  of  Heaven."} 


54  NATURAL    PHILOSOPHY. 

("?.}  A  boy  raised  his  kice  in  the  night,  with  a  lantern  attached  to  it 
Ihifoiunately,  the  string  which  attached  the  lantern  broke,  and  the  lanton 
foil  U»  the  ground  in  6  seconds.  How  high  was  the  'rite  1  Ans.  r>76/i5. 

175.  RETARDED  MOTION  OF  BODIES  PROJECTED  UPWARDS.  —  All  the 
circumstances  attending  the  accelerated  descent  of  falling  bodies  are 
exhibited  when  a  body  is  projected  upwards,  but  in  a  reversed  order. 


176'  To  determine  the  height  to  which  a 
height  to  which  body,    projected   upwards,    will   rise,   with   a 

^etfed  upwards  °iven  velocitJ>  Jt  is  onlJ  necessary  to  deter- 
with  a  given     mine  the  height  from  which  a  body  would  fall 

t0  aC(luire  the  Same  velo(% 


177.  Thus,  if  it  be  required  to  ascertain  how  high  a  body  would 
rise  when  projected  upwards  with  a  force  sufficient  to  carry  it  It* 
feet  in  the  first  second  of  time,  we  reverse  the  series  of  numbers 
1G-4-  48  +  804-112-[-144  [see  table  on  page  52],  and,  reading 
them  backward,  144  -\-  112  -4-  80  -j-  48  -f-  16,  we  find  their  sum  to  be 
400  feet,  and  the  time  employed  would  be  5  seconds. 

How  does  the 

time  of  the  as-      178.  The  time  employed  in  the  ascent  and 

^m°{rewith    descent   of  a   body   projected   upwards   will, 
the  time  of  its  therefore,  always  be  equal. 
descent  1 

Questions  for  Soliition 

(1.)  Suppose  a  cannon-ball,  projected  perpendicularly  upwards,  returneo 
to  the  ground  in  18  seconds  ;  how  high  did  it  ascend,  and  what  is  the  velocity 
of  projection  1  Ans  1'296/i.  ;  272  ft.  1st  sec. 

(2.)  How  high  will  a  stone  rise  which  a  man  throws  upward  with  a  forea 
(Sufficient  to  carry  it  48  feet  during  the  first  second  of  time  1  Ans.  $4Jt. 

(3.)  Suppose  a  rocket  to  ascend  with  a  velocity  sufficient  to  carry  it  17€ 
feet  during  the  first  second  of  time  ;  how  high  will  it  ascend,  and  what 
tiiue  will  it  occupy  in  its  ascent  and  descent  1  Ans.  576.A  ;  12  sec. 

(4.)  A  musket-ball  is  thrown  upwards  until  it  reaches  the  height  of  400 
ftet.  How  long  a  time,  in  seconds,  will  it  occupy  in  its  ascent  and  descent, 
and  what  space  does  it  ascend  in  the  first  second  T  Ans.  10  sec.  ;  144  A 

(5.)  A  sportsman  shoots  a  bird  flying  in  the  air,  and  the  bird  is  3 
.'cconds  in  falling  to  the  ground.  How  high  up  was  the  bird  when  he  was 
ahot  ?  -Aw*  144A 

(G.)  How  long  time,  in  seconds,  would  it  take  a  ball  to  reach  an  object 
3000  feet  above  the  surface  of  the  earth,  provided  that  the  ball  be  projected 
with  a  force  sufficient  only  to  reach  the  object  ]  Ans.  17.67  sec.  + 

179.  COMPOUND  MOTION.  —  Motion  may  be  produced 
either  by  a  single  force  or  by  the  operation  of  two  or  mow 


MECHANICS.  55 

*n  what  direc-       180.  Simple  Motion  is  the  motion  of  a  body 

'ion  is  the  mo-  impelle(}  by  a  single  force,  and  is  always  in  a 
f  ton  of  a  body  r  *  °  7  .  • 

impelled  by  a  straight  line  in  the  same  direction  with  the 
single  force?  force  that  acts. 

What  is  Com-  181.  Compound  Motion  is  caused  by  the 
pound  Motion?  Operation  Of  two  or  more  forces  at  the  same 
time. 

When  a  body 

is  struck  by  two      182.  When  a  body  is  struck  by  two  equal 

equalforces,  m  forces<  jn  opposite  directions,  it  will  remain  at 
opposite  direc- 
tions, h)w  will  rest. 
it  move  ? 

183.  If  the  forces  be  unequal,  the  body  will  move  with  dimin- 
ished force  in  the  direction  of  the  greater  force.  Thus,  if  a  body 
with  a  momentum  of  9  be  opposed  by  another  body  with  a  momen- 
tum of  6,  both  will  move  with  a  momentum  of  3  in  the  direction  of 
the  greater  force. 

How  will  a  184-  A  bodJ>  struck  by  two  forces  in  dif- 
body  move  ferent  directions,  will  move  in  a  line  between 
^forrceskiny  them>  in  the  direction  of  the  diagonal  of  a 
different  direc-  parallelogram,  having  for  its  sides  the  lines 
through  which  the  body  would  pass  if  urged 
by  each  of  the  forces  separately. 

How  will  the 

body  move,  if       185.  When   the   forces   are   equal   and  at 

ansles  to  each  °ther>  tha  forty  wil1 


right  angles  to  move  in  the  diagonal  of  a  square. 
each  other  ? 

186.  Let  Fig.  11  represent  a  ball  struck  by  Fig  n 
the  two  equal  forces  X  and  Y.  In  this  figure 
the  forces  are  inclined  to  each  other  at  an  angle 
of  90°,  or  a  right  angle.  Suppose  that  the 
force  X  would  send  it  from  C  to  B,  and  the 
force  Y  from  C  to  D.  As  it  cannot  obey  both, 
it  will  go  between  them  to  A,  and  the  line  C  A, 


56  NATURAL    PHILOSOPHY. 

through  which  it  passes,  is  the  diagonal  of  the  square,  A  B  C  D 
This  line  also  represents  the  resultant  of  the  two  forces. 

The  time  occupied  in  its  passage  from  C  to  A  will  be  th€ 
same  as  the  force  X  would  require  to  send  it  to  B,  or  the  force 
Y  to  send  it  to  D. 

How  will  a  -ir>fr     Tf 

body  move  187-  If  two  unequal  forces  act  at   right 

under  the  influ-  angles  to  each  other  on  a  body,  the  body  will 
enceoftwoun-  .        .        ,.         .          i  .f        T  /    r« 

equal  forces  at  move   in   the  direction  of  the  diagonal  of  a 

right  angles  to  rectangle, 
each  other  ? 

Explain  Fig.      188.  Illustration.  —  In  Fig.  12  the   ball  C   ifi 

represented  as  acted  apon  by 
two  unequal  forces,  X  and  Y.  The  force  X 
would  send  it  to  B,  and  the  force  Y  to  D.  As 
it  cannot  obey  both,  it  will  move  in  the  direc- 
tion C  A,  the  diagonal  of  the  rectangle  A  B  C  D.  ^ '  B 


.  act  in  ** 

direction  of  any  other      direction  of  an   acute   or   an   obtuse 

than  a  right  angle?  j      th     bodjr  will  move  m  the   di_ 

How  will  a  body  move  if       °  t  '       ^          J 

the  forces  act  in  the  di-    rection  of  the  diagonal  of  a  para'Jlelo- 

rection  of  an  acute  or       ffram 

obtuse  angle  ? 

Explain        190.    Illustration.  —  In   figure  13  the  ball  C  ib 

Fig.  13.    supposed  to  be  influenced  by  two  Flg  13> 

forces,  one  of  which  would  send  it  to  B  and 

the  other  to  D,  the  forces  acting  in  tho 

direction  of  an  acute  angle.     The  ball  will, 

therefore,  move  between  them  in  the  line 

C  A,  the  longer  diagonal  of  the  paralleiogm^  A  B  C  D. 

191.  The  same  figure  explains  the  motion  of  a  ball  when  ihc 
two  forces  act  in  the  direction  of  an  obtuse  angle. 

192.  Illustration. —  ."he  ball  D,  ui.der  tbe  intlucnce  of  twc 


MECHANICS.  57 

forces,  one  of  which  would  send  it  to  C,  and  the  other  to  A, 
which,  it  will  be  observed,  is  in  the  direction  of  an  obtuse 
angle,  will  proceed  in  this  case  to  B,  the  shorter  diagonal  of  the 
parallelogram  A  B  C  D. 

[N.  B.  A  parallelogram  containing   acute   and  obtuse   angles   has   lw 
diagonals,  the  one  which  joins  the  acute  angles  being  the  longer.] 

What  is  Re-         193.  Resultant  Motion  is  the  effect  or  re 
suit  ant  Mo-  ,^     P  ,  .  ,    ,  . 

tion  ?  su^  0*  two  motions  compounded  into  one. 

194.  If  two  men  be  sailing  in  separate  boats,  in  the  same 
direction,  and  at  the  same  rate,  and  one  toss  an  apple  to  the 
other,  the  apple  would  appear  to  pass  directly  across  from  one 
to  the  other,  in  a  line  of  direction  perpendicular  to  the  side  of 
each  boat.  But  its  real  course  is  through  the  air  in  the  diag- 
onal of  a  parallelogram,  formed  by  the  lines  representing  the 
course  of  each  boat,  and  perpendiculars  drawn  to  those  lines 
from  the  spot  where  each  man  stands  as  the  one  tosses  and  the 
Explain  other  catches  the  apple.  In  Fig.  14 
Fig.  14.  the  lines  A  B  and  C  D  represent  the 
course  of  each  boat.  E  the  spot  where  the  man  A 
stands  who  tosses  the  apple ;  while  the  apple  is  c 
in  its  passage,  the  boats  have  passed  from  E 
and  G  to  II  and  F  respectively.  But  the  apple,  having  a 
motion,  with  the  man,  that  would  carry  it  from  E  to  H,  and 
likewise  a  projectile  force  which  would  carry  it  from  E  to  G, 
cannot  obey  them  both,  but  will  pass  through  the  dotted  line 
E  F,  which  is  the  diagonal  of  the  parallelogram  E  G  F  H.* 

How  can  we          195.  When  a  body  is  acted  upon  by  three  <v 
n  of^he  more  f°rces  a*  the  same  time,  we  may  take  any 


*  On  the  principle  of  resultant  motion,  if  two  ships  in  an  engagement  be 
lailing  before  the  wind,  at  equal  rates,  the  aim  of  the  gunners  will  be 
exactly  as  though  they  both  stood  still.  But,  if  the  gunner  fire  from  a  ship 
standing  still  at  another  under  sail,  or  a  sportsman  fire  at  a  bird  on  the 
wing,  each  should  take  his  aim  a  little  forward  of  the  mark,  because  the 
ship  and  the  bird  will  pass  a  little  forward  while  the  shot  is  passing  to 
them. 


68  NATURAL    PHILOSOPHY. 

motion  when  two  of  them  alone,  and  ascertain  the  resultant  of 

iuen°cedby  '**  those  two'  and  then  emPloJ  tho  resultant  as  a  nev» 

hree  or  more  force,  in  conjunction  with  the  third,*  &c. 


orces 


What  is  Cir-       196.   CIRCULAR  MOTION.  —  Circular  Mo- 
cular  Motion  7  ^on  js  motion  around  a  central  point. 

What  causes  ^-^'  Circular  m°tion  is  caused  by  the  con- 
Circular  Mo-  tinued  operation  of  two  forces,  by  one  of  which 
the  body  is  projected  forward  in  a  straight 
line,  while  the  other  is  constantly  deflecting  it  towards  a 
fixed  point.  [See  No.  184.] 

198.  The  whirling  of  a  ball,  fastened  to  a  string  held  by  the 
hand,  is  an  instance  of  circular  motion.  The  ball  is  urged  by  two 
forces,  of  which  one  is  the  force  of  projection,  and  the  other  the 
string  which  confines  it  to  the  hand.  The  two  forces  act  at  right 
angles  to  each  other,  and  (according  to  No.  184)  the  ball  will  move 
in  the  diagonal  of  a  parallelogram.  But,  as  the  force  which  con- 
anes  it  to  the  hand  only  keeps  it  within  a  certain  distance,  without 
drawing  it  nearer  to  the  hand,  the  motion  of  the  ball  will  be  through 
the  diagonals  of  an  indefinite  number  of  minute  parallelograms, 
formed  by  every  part  of  the  circumference  of  the  circle. 

How  many  199.  There  are  three  different  centres  which 

centres  re-  require  to  be  distinctly  noticed  ;  namely,  the 
noticed  in  Me-  Centre  of  Magnitude,  the  Centre  of  Gravity, 
chanics?  an(j  the  Centre  of  Motion. 


*  The  resultant  of  two  forces  is  always  described  by  the  third  side  of  ft 
triangle,  of  which  the  two  forces  may  be  represented,  in  quantity  and 
direction,  by  the  other  two  sides.  When  three  forces  act  in  the  direction 
of  the  three  sides  of  the  same  triangle,  the  body  will  remain  at  rest. 

When  two  forces  act  at  right  angles,  the  resultant  will  form  the  hypothe 
nuse  of  a  right-angled  triangle,  either  of  the  sides  of  which  may  be  found, 
when  the  two  others  are  given,  by  the  common  principles  of  arithmetic  or 
geometry. 

From  what  has  now  been  stated,  it  will  easily  be  seen,  that  if  any  number 
of  forces  whatever  act  upon  a  body,  and  in  any  directions  whatever,  the 
resultant  of  them  all  may  easily  be  found,  and  this  resultant  will  be  their 
mechanical  equivalent.  Thus,  suppose  a  body  be  acted  upon  at  the  same 
time  by  six  forces,  represented  by  the  letters  A,  B,  0,  D,  E,  F.  First  find 
the  resultant  of  A  and  B  by  the  law  stated  in  No.  184,  and  call  this  resultant 
Q.  In  the  same  manner,  find  the  resultant  of  G  and  C,  calling  it  H."  Then 
find  the  resultant  of  H  and  D,  and  thus  continue  until  each  of  the  forces  be 
found  and  the  last  resultant  will  be  the  mechanical  equivalen*  of  the  whole 


MECHANICS. 


51) 


What  is  ike 
Centre  of 
Magnitude . 

What  is  the 
Centre  of 
Gravity  ? 

What  is  the 
Centre  of 
Motion  ? 


200.  The  Centre  of  Magnitude  is  the  central 
point  of  the  bulk  of  a  body. 

201.  The  Centre  of  Gravity  is  the  point 
about  which  all  the  parts  balance  each  other.   ' 

202.  The   Centre  of  Motion  is  the   point 
around  which  all  the  parts  of  a  body  move. 


203.  When  the  body  is  not  of  a  size  nor 
shape  to  allow  every  point  to  revolve  in  the 
same  plane,  the  line  around  which  it  revolves 
is  called  the  Axis  of  Motion.* 


What  is  the 
Axis  of  Mo- 
tion ? 


204.  The  centre  or  the  axis  of  motion 
generally  supposed  to  be  at  rest. 


is 


Does  the  cen- 
tre or  the  axis 
of  motion  re- 
volve ? 

205.  Thus  the  axis  of  a  spinning-top  is  stationary,  while  ever\ 
other  part  is  in  motion  around  it.  The  axis  of  motion  and  the 
centre  of  moti3n  are  terms  which  relate  only  to  circular  motion. 

What  are  Cen-  206.  The  two  forces  by  which  circular 
tral Forces?  motion  is  produced  are  called  Central  Forces. 
Their  names  are,  the  Centripetal  Force  and  the  Centrifugal 
Force.f 

207.  The  Centripetal  Force  is  that  which 

confines  a  body  to  the  centre  around  which  it 

revolves. 


What  is  the 
Centripetal 
Force  * 


208.  The  Centrifugal  Force  is  that  which 


What  is  the 

Force?  impels  the  body  to  fly  off  from  the  centre. 


*  Circles  may  have  a  centre  of  motion  ;  spheres  or  globes  have  an  axis 
of  motion.  Bodies  that  have  only  length  and  breadth  may  revolve  around 
their  own  centre,  or  around  axes  ;  those  that  have  the  three  dimensions  of 
length,  breadth  and  thickness,  must  revolve  around  axes. 

t  The  word  centripetal  means  seeking  the  centre,  and  centrifugal  means 
flying  ftom  the  centre.  In  circular  motion  these  two  forces  constantly 
balance  each  other  ;  otherwise  the  revolving  body  will  either  «7proaob 
the  centre,  o:  recede  from  it,  according  as  the  centripetal  or  centrifugal 
force  is  the  stronger. 

3 


60  NATURAL    PHILOSOPHY. 

209.  If  the  centrifugal  force  of  a  revolving 
destroyed,  the  body  will  immediately 
trifugal  force  approach  the  centre  which  attracts  it  ;  but  if 
be  destroyed?  fae  centripetal  force  be  destroyed,  the  body 
will  fly  off  in  the  direction  of  a  tangent  to  the  curve  which 
it  describes  in  its  motion.* 

'210.  Thus,  when  a  mop  filled  with  water  is  turned  swiftly  round 
by  the  handle,  the  threads  which  compose  the  head  will  fly  off  from 
the  centre  ;  but,  being  confined  to  it  at  one  end,  they  cannot  part  from 
it  ;  while  the  water  they  contain,  being  unconfined,  is  thrown  off 
in  straight  lines. 


21L  The  Parts  of  a  bod7  which  are 
around  its        from   the   centre   of   motion   move   with   the 

greatest  velocity  ;  and  the  velocity  of  all  the 
parts  move  with  parts  diminishes  as  their  distance  from  the 

axis  of  motiou  diminishes- 


Explain         212.  Fig.  15  represents  the  vanes  of  a  windmill. 

Fig.  15.    The  circles  denote  the  paths  in  which   the  different 

parts  of  the  vanes  move.     M  is  the  centre  F.    lft 

or  axis  of  motion   around  which   all    the 

parts  revolve.     The  outer  part  revolves  in 

the  circle  D  E  F  G,  another  part  revolves 

in  the  circle  H  I  J  K,  and  the  inner  part  in 

the  circle  L  N  0  P.     Consequently,  as  they 

all  revolve  around  M  in  the  same  time,  the 

velocity  of  the  parts  which  revolve  in  the 

outer  circle  is  as  much  greater  than  the  velocity  of  the  parts 

which  revolve  in  the  inner  circle,  L  N  0  P,  as  the  diameter  oi' 

the  outer  circle  is  greater  than  the  diameter  of  the  inner. 


*  The  centrifugal  force  is  proportioned  to  the  square  of  the  velocity  of  a 
moving  body.  Hence,  a  cord  sufficiently  strong  to  hold  a  heavy  body 
revolving  around  a  fixed  centre  at  the  rate  of  fifty  feet  in  a  second,  woulo 
require  to  have  ics  strength  increased  four-fold,  to  hold  the  same  ball,  if  it* 
relosity  should  be  doubled. 


MECHANICS.  fjl 

In  the  daily  rcvolu-  213.  As  the  earth  revolves  round  its 
Hon  of  the  earth  .,  P  ,,  P  A,  ,.  .,, 

ar<nm*iu<»t>na*it,  axi93  ^  follows,  from  the  preceding  illus- 
what  parts  of  the  tration,  that  the  portions  of  the  earth 
earth  move  most  •,  .  ,  .  -,,  .-> 

slowly,  and  what     which  move  most  rapidly  are  nearest  to  the 

purls  most  rapidly  ?  equator,  and  that  the  nearer  any  portion 
of  the  earth  is  to  the  poles  the  slower  will  be  its  motion. 

What  is  re-          214.   Curvilinear  motion  requires  the  action 
two  f°rces  ;  f°r  tne  impulse,  of  one  single 


curvilinear       force   always  produces   motion  in  a  straight 
motion?  and     •.. 
why?  lme' 

What  effect  215.  A  body  revolving  rapidly  around  its 
u°gal  ^  force  on  longer  axis,  if  suspended  freely,  will  gradually 
a  body  revolv-  change  the  direction  of  its  motion,  and  revolve 

ing  around  its  ,  .        , 

longer  axis?     around  its  shorter  axis. 

This  is  due  to  the  centrifugal  force,  which,  impelling  the  parts 
from  the  centre  of  motion,  causes  the  most  distant  parts  to  revolve 
in  a  larger  circle.* 


*  This  law  is  beautifully  illustrated  by  a  simple  apparatus,  in  which  a 
hook  is  made  to  revolve  rapidly  by  means  of  multiplying  wheels.  Let  an 
oblate  spheroid,  a  double  cone,  or  any  other  solid  having  unequal  axes,  be 
suspended  from  the  hook  by  means  of  a  flexible  cord  attached  to  the  ex- 
tremity of  the  longer  axis.  If,  now,  it  be  caused  rapidly  to  revolve,  it  will 
immediately  change  its  axis  of  motion,  and  revolve  around  the  shorter  axis. 

The  experiment  will  be  doubly  interesting  if  an  endless  chain  be  sus- 
pended from  the  hook,  instead  of  a  spheroid.  So  soon  as  the  hook  with  the 
chain  suspended  is  caused  to  revolve,  the  sides  of  the  chain  are  thrown  out- 
ward by  the  centrifugal  force,  until  a  complete  ring  is  formed,  and  then  the 
circular  chain  will  commence  revolving  horizontally.  This  is  a  beautiful 
illustration  of  the  effects  of  the  centrifugal  force.  An  apparatus,  with  u 
chain  and  six  bodies  of  different  form,  prepared  to  be  attached  to  the  multi- 
plying whoels  in  the  manner  described,  accompanies  most  sets  of  philo- 
sophical apparatus. 

Attached  to  the  same  apparatus  is  a  thin  hoop  of  brass,  prepared  for  con 
nexion  with  the  multiplying  wheels.  The  hoop  is  made  rapidly  to  revolve 
around  a  vertical  axis,  loose  at  the  top  and  secured  below.  So  soon  as  tho 
hoop  begins  to  revolve  rapidly,  the  horizontal  diameter  of  the  ring  begins 
to  increase  and  the  vertical  diameter  to  diminish,  thus  exhibiting  the 
manner  in  which  the  equatorial  diameter  of  a  revolving  body  is  lengthened, 
and  the  polar  diameter  is  shortened,  by  reason  of  the  centrifugal  force. 
The  daily  revolution  of  the  earth  around  its  axis  has  produced  this  effect, 
go  that  the  equatorial  diameter  is  at  least  twenty-six  miles  longer  than  thu 
polar,  lii  those  planets  that  revolve  faster  thau  the  earth  the  effect  is  still 


()2  NATURAL   PHILOSOPHY. 

What  is  Pro-      216.  PROJECTILES.  —  Projectiles  is  a  brancl 
jectiles.  Df  Mechanics  which  treats  of  the  motion  of 

bodies  thrown  or  driven  by  an  impelling  force  above  the 
surface  of  the  earth. 

What  is  a  217.  A  Projectile  is  a  body  thrown  *nto  the 

Projectile?       ajrj — as  a  rocket,  a  ball  from  a  gun,  or  a 
stone  from  the  hand. 

The  force  of  gravity  and  the  resistance  of  the 
How  are  pro-       .  ,     f         J  .          . 

iectiles  affected  air  cause  projectiles  to  lorm  a  curve  both  in  their 

in  their  mo-       ascent   and  descent ;    and,   in  descending,  their 
motion  is   gradually    changed   from   an  oblique 
towards  a  perpendicular  direction. 

Explain        218.  In  Fig.  16  the  force  of  projection  would  carry 
Fig.  16.    a  ball  from  A  to  D,  while  gravity  would  bring  it  to 
0.     If  these  two  forces  alone  prevailed,  the 
ball  would  proceed  in  the  dotted  line  to  B.  D 
But,  as  the  resistance  of  the  air  operates  in 
direct  opposition  to  the  force  of  projection, 
instead  of  reaching  the  ground  at  B,  the  ball  B 
will  fall  somewhere  about  E.^ 

What  is  the          219.  When   a  body  is  thrown  fig.  11. 

course  of  a  in  a  horizontal  direction,  or  up- 

J&ZgZa  ™rds  or  downwards,  obUyuely,  its 

horizontal  course  will  be  in  the  direction  of 

direction?  a  curve-lme,  called  a  parabola*  A 


more  striking,  as  is  the  case  with  the  planet  Jupiter,  whose  figure  is  nearlj 
that  of  an  oblate  spheroid. 

The  developments  of  Geology  have  led  some  writers  to  the  theory  that 
the  earth,  during  one  period  of  its  history,  must  have  had  a  different  axir 
of  motion  ;  but  it  will  be  exceedingly  difficult  to  reconcile  such  a  theory  i 
the  law  of  rotations  which  has  now  been  explained,  especially  as  a  muc 
more  rational  explanation  can  be  given  to  the  phenomena  on  which  tl «• 
theory  was  built. 

*  It  is  calculated  that  the  resistance  of  the  air  to  a  cannon-ball  of  Im- 
pounds' weight,  with  the  velocity  of  two  thousand  feet  in  a  second,  is  moi « 
than  equivalent  to  sixty  times  the  weight  of  the  ball. 

\  The  science  of  gunnery  is  founded  upon  the  laws  relating  to  project! let 


MECHANICS.  68 

(see  Fig.  17; ;  but  when  it  is  thrown  perpendicularly  upwards 
or  downwai  Is,  it  will  move  perpendicular!}7,  because  the  force 
of  projection  and  that  of  gravity  are  in  the  same  line  of 
direction. 

The  force  of  gunpowder  is  accurately  ascertained,  and  calculations  are 
predicated  upon  these  principles,  which  enable  the  engineer  to  direct  his 
guns  in  such  a  manner  as  to  cause  the  fall  of  the  shot  or  shells  in  the  very 
spot  where  he  intends.  The  knowledge  of  this  science  saves  an  immense 
expenditure  of  ammunition,  which  would  otherwise  be  idly  wasted,  without 
producing  any  effect.  In  attacks  upon  towns  and  fortifications,  the  skilful 
engineer  knows  the  means  he  has  in  his  power,  and  can  calculate,  with 
great  precision,  their  effects.  It  is  in  this  way  that  the  art  of  war  has  been 
elevated  into  a  science,  and  much  is  made  to  depend  upon  skill  which, 
previous  to  the  knowledge  of  these  principles,  depended  entirely  upon 
physical  power. 

The  force  with  which  balls  are  thrown  by  gunpowder  is  measured  by  an 
instrument  called  the  Ballistic  pendulum.  It  consists  of  a  large  block  of 
wood,  suspended  by  a  rod  in  the  manner  of  a  pendulum.  Into  this  block 
the  balls  are  fired,  and  to  it  they  communicate  their  own  motion.  Now, 
the  weight  of  the  block  and  that  of  the  ball  being  known,  and  the  motion 
or  velocity  of  the  block  being  determined  by  machinery  or  by  observation, 
the  elements  are  obtained  by  which  the  velocity  of  the  ball  may  be  found  ; 
for  the  weight  of  the  ball  is  to  the  weight  of  the  block  as  the  velocity  of  the  block  is 
to  the  velocity  of  the  ball.  By  this  simple  apparatus  many  facts  relative  to 
the  art  of  gunnery  may  be  ascertained.  If  the  ball  be  fired  from  the  same 
gun,  at  different  distances,  it  will  be  seen  how  much  resistance  the  atmo- 
sphere opposes  to  its  force  at  such  distances.  Rifles  and  guns  of  smooth 
bores  may  be  tested,  as  well  as  the  various  charges  of  powder  best  adapted 
to  different  distances  and  different  guns.  These,  and  a  great  variety  of 
other  experiments,  useful  to  the  practical  gunner  or  sportsman,  may  be 
made  by  this  simple  means. 

The  velocity  of  balls  impelled  by  gunpowder  from  a  musket  with  a 
eommon  charge  has  been  estimated  at  about  1650  feet  in  a  second  of  time, 
when  first  discharged.  The  utmost  velocity  that  can  be  given  to  a  cannon- 
ball  is  2000  feet  per  second,  and  this  only  at  the  moment  of  its  leaving  Ue 
gun. 

In  order  to  increase  tht  velocity  from  1650  to  2000  feet,  one-half  more 
powder  is  required  ;  and  even  then,  at  a  long  shot,  no  advantage  is  gained, 
since,  at  the  distance  of  500  yardsv  the  greatest  velocity  that  can  be  ob- 
tained is  only  1200  or  1300  feet  per  second.  Great  charges  of  powder  are, 
therefore,  not  only  useless,  but  dangerous  ;  for,  though  they  give  little 
additional  force  to  the  ball,  they  hazard  the  lives  of  many  by  their  liability 
to  burst  the  gun. 

Experiment  has  also  shown  that,  although  long  guns  give  a  greater 
velocity  to  the  shot  than  short  ones,  still  that,  on  the  whole,  short  ones  are 
preferable  ;  and,  accordingly,  armed  ships  are  now  almost  invariably 
furnished  with  short  guns,  called  carronades. 

The  length  of  sporting  guns  has  also  been  greatly  reduced  of  late  years 
Fcrmerly,  the  barrels  were  from  four  to  six  feet  in  length  ;  but  the  best 
fowling-pieces  of  the  present  day  have  barrels  of  two  feet  or  two  and  a  half 
only  in  length  Guns  of  about  this  length  are  now  universally  employed 
for  such  game  as  woodcocks,  partridges,  grouse,  and  such  birds  as  are  taken 
on  the  wing,  with  the  exceptions  of  duckg  aud  wild  geese,  which  require 
and  heavier  guns 


64  NATURAL    PHILOSOPHY. 


A  bal1  thr°Wn   ln   a   h01  Z°ntal  directlr)n 

izontal  pro-    is  influenced  by  three  forces  ;  namely,  first,  the 

whateffect^do    ^orce  °^  Projec^on  (which  gives  it  a  horizontal 

they  produce?   direction)  ;   second,  the  resistance  of  the  air 

through  which  it  passes,  which  diminishes  its  velocity,  with- 

out changing  its  direction  ;  and  third,  the  force  of  gravity, 

which  finally  brings  it  to  the  ground. 

How   is  the 

gravity  af-       221.  The  force  of  gravity  is  neither  increased 

fectedby  the    nor  diminished  by  the  force  of  projection.* 
/  rce  of  pro- 
jection " 

Explain        222.  Fig.  18  represents  a  Fig.  18 

l£'  '  cannon,  loaded  with  a  ball, 
and  placed  on  the  top  of  a  tower,  at 
such  a  height  as  to  require  just  three 
seconds  for  another  ball  to  descend  per- 
pendicularly. Now,  suppose  the  can- 
non to  be  fired  in  a  horizontal  direc- 

tion, and  at  the  same  instant  the  other  ball  to  be  dropped  towards 
the  ground.  They  will  both  reach  the  horizontal  line  at  the 
base  of  the  tower  at  the  same  instant.  In  this  figure  C 
a  represents  the  perpendicular  line  of  the  falling  ball.  C  b  is 
the  curvilinear  path  of  the  projected  ball,  3  the  horizontal  line 
at  the  base  of  the  tower.  During  the  first  second  of  time,  the 
"ailing  ball  reaches  1,  the  next  second  2,  and  ajfc  the  end  of  the 

*  The  action  of  gravity  being  always  the  same,  the  shape  of  the  curve  of 
every  projectile  depends  on  the  velocity  of  its  motion  ;  but,  whatever  this 
velocity  be,  the  moving  body,  if  thrown  horizontally  from  the  same  eleva- 
tion, will  reach  the  ground  at  the  same  instant.  Thus,  a  ball  from  a  cannon, 
with  a  charge  sufficient  to  throw  it  half  a  mile,  will  reach  the  ground  at  the 
same  instant  of  time  that  it  would  had  the  charge  been  sufficient  to  throw  it 
one,  two,  or  six  miles,  from  the  same  elevation.  The  distance  to  which  a 
ball  will  be  projected  will  depend  entirely  on  the  force  with  which  it  ig 
thrown,  or  on  the  velocity  of  its  motion.  If  it  moves  slowly,  the  distance 
will  be  short  ;  if  more  rapidly,  the  space  passed  over  in  the  sa.ine  time 
will  be  greater  ;  but  in  both  cases  the  descent  of  the  ball  towards  the  earth 
in  the  same  time,  will  be  the  same  number  of  feet,  whether  it  UK  ?es  fast  or 
slow,  <>r  eveu  whether  it  uiovo  forward  at  all  or  nut. 


MECHANICS.  65 

second  it  strikes  the  ground.  Meantime,  that  projected 
from  the  cannon  moves  forward  with  such  velocity  as  to  reach 
4  at  the  saoe  time  that  the  falling  ball  reaches  1.  But  the 
projected  ball  falls  downwards  exactly  as  fast  as  the  other,  since 
ife  meets  the  line  1  4,  which  is  parallel  to  the  norizon,  at  the  same 
instant.  During  the  next  second  the  ball  from  the  cannon 
reaches  5,  while  the  other  falls  to  2,  both  having  an  equal  de- 
scent. During  the  third  second  the  projected  ball  will  have 
spent  nearly  its  whole  force,  and  therefore  its  downward  motion 
will  be  greater,  while  the  motion  forward  will  be  less  than  before. 

What  effect      223.  Hence  it  appears  that  the  horizonta* 

has  the  pro-  moflon  ^oes  not  interfere  with  the  action  of 
jectile  jorce  J  . 

on  gravity*     gravity,  but  that  a  projectile  descends  with 

the  same  rapidity  while  moving  forward  that  it  would 
if  it  were  acted  on  by  gravity  alone.  This  is  the  neces- 
sary result  of  the  action  of  two  forces. 

What  is  the  224.  The  Random  of  a  projectile  is  the  horizontal 
Random  of  a  ,.  .  .  .  ,, 

vrojsctile  ?          distance  from  the  place  whence  it  is  thrown  to  the 

place  where  it  strikes. 

At  what  angle  225.  The  greatest  random  takes  place  at  au 
est  random  ~  an^e  °f  45  degrees;  that  is,  when  a  gun  ia 
take  place  ?  pointed  at  this  angle  with  the  horizon,  the  ball  is 
thrown  to  the  greatest  distance. 

What^  will^  be      Let  Fig.  19  represent  a  gun  or  Fig.  19 

a  carronade,  from  which  a  ball 

is  thrown  at  an  angle  of  45  de- 
above  45  de-  grees  witn  the  horizon.  If 

the  ball  be  thrown  at  any  angle 
above  45  degrees,  the  random  will  be  the  same 
as  it  would  be  at  the  same  number  of  degrees  below  45  degrees.* 

*  A  knowledge  of  this  fact,  and  calculations  predicated  on  it,  enables  the 
engineer  so  to  direct  his  guns  as  to  reach  the  object  of  attack  when  with  it 
tue  range  of  shot. 


66  NATURAL    PHILOSOPHY. 

What  is  tfu      226.  CENTRE  OF  GRAVITY.  —  It  has  already 

Centre  of         been  stated   ^  Nos>    10g   &    11Qj    that  tht 

Gravity  of  a 

body?  Centre  of  Gravity  of  a  body  is  the  point 

around  win  -h  all  the  parts  balance  each  other.  It  is  in 
other  words,  the  centre  of  the  weight  of  a  body.  (See 
Appendix,  par.  1404.) 


e       227<  Tlie  Centre  of  Magnitude  is  the  central 
Magnitude  *    point  of  the  bulk  of  a  body. 

Where  is  the      228.  When  a  body  is  of  uniform  density,  the 

centre  of        centre  of  gravity  is  in  the  same  point  with  the 

gravity  oj  a 

body  ?  centre  of  magnitude.     But  when  one  part  of  the 

body  is  cor  \posed  of  heavier  materials  than  another  part,  the 
centre  of  gravity  (being  the  centre  of  the  weight  of  the  body) 
no  longer  corresponds  with  the  centre  of  magnitude. 

Thus  the  centre  of  gravity  of  a  cylinder  plugged  with  lead  is  no. 
in  the  same  point  as  the  centre  of  magnitude. 

If  a  body  be  composed  of  different  materials,  not  united  in  chemical 
combination,  the  centre  of  gravity  will  not  correspond  with  the  centre 
of  magnitude,  unless  all  the  materials  have  the  same  specific  gravity. 

When  will  a  229.  When  the  centre  of  gravity  of  a  body  is 
body  stand  supported,  the  body  itself  will  be  supported  ; 

and  when  will  '  J  ' 

H  fall  ?  but  when  the  centre  of  gravity  is  unsupported, 

the  body  will  fall. 

What  is  the       230.  A  line  drawn  from  the  centre  of  tfrav- 

L-neofDirec-  m  ,       .      .  . 

tion?  ity,  perpendicularly  to  the  horizon,  is  called 

tli3  Line  of  Direction. 

231.  The  line  of  direction  is  merely  a  line  indicating  the  path 
which  the  centre  of  gravity  would  describe,  if  the  body  were  per 
mitted  to  fall  freely. 


MECHANICS.  07 

Wen  will  a      232.  When  the  line  of  direction  falls  within 

bandwhmwill  the  baSG  *  °f  ai^  b°d^'  the  b°d^  Vri11  Stand  5  but 
it  fall?  when  that  line  falls  outside  of  the  base,  the 

body  will  fall,  or  be  overset. 

E^lain        2*3.   (1.)  Fig.  21  represents  a  loaded        Fig.  21. 

Pt8-  2L    wagon  on  the  declivity  of  a  hill.     The 

line  C  F  represents  a  horizontal  line,  D  E  the  base 

of  the  wagon.     If  the  wagon  be  loaded  in  such  a 

manner  that  the  centre  of  gravity  be  at  B,  the  per-    c  p 

pendicular  B  D  will  fall  within  the  base,  and  the  wagon  will 

stand.     But  if  the  load  be  altered  so  that  the  centre  of  gravity 

be  raised  to  A,  the  perpendicular  A  C  will  fall  outside  of  the 

base,  and  the  wagon  will  be  overset.     From  this  it  follows  that 

a  wagon,  or  any  carriage,  will  be  most  firmly  supported  when 

the  line  of  direction  of  the  centre  of  gravity  falls  exactly  between 

the  wheels ;  and  that  is  the  case  on  a  level  road.     The  centre  of 

gravity  in  the  human  body  is  between  the  hips,  and  the  base  is 

the  feet. 

234.  So  long  as  we  stand  uprightly,  the  line  of  direction  falls 
within  this  base.  When  we  lean  on  one  side,  the  centre  of  gravity 
not  being  supported,  we  no  longer  stand  firmly. 

How  does  a  235.  A  rope-dancer  performs  all  his  feats  of  agil 
r°erformCehis  ^r  b^  dexterously  supporting  the  centre  of  gravity 
feats  of  agil-  For  this  purpose,  he  carries  a  heavy  pole  in  his 
*ty  *  hands,  which  he  shifts  from  side  to  side  as  he  alters 

hi&  position,  in  order  to  throw  the  weight  to  the  side  which  is 
deficient ;  and  thus,  in  changing  the  situation  of  the  centre  of 
gravity  he  keeps  the  line  of  direction  within  the  base,  and  he 
will  not  fall.t 

*  Tha  base  of  a  body  is  its  lowest  side.     The  base  ^8'  2Q> 

of  a  bod/  standing  on  wheels  or  legs  is  represented  by 
lines  drawn  from  the  lowest  part  of  one  wheel  or  leg 
to  the  lowest  part  of  the  other  wheel  or  leg. 

Thus,  in  Figs.  20  and  21,  D  E  represents  the  base  of 
the  wagon  and  of  the  table. 

t  The  shepherds  in  the  south  of  France  afford  an  interesting  instance  of 
the  application  of  the  art  of  balancing  to  the  common  business  of  life 
Tiieso  men  walk  on  stilts  from  three  to  four  feet  high,  and  their  children 

3* 


NATURAL    PHILOi: :  PI  Y. 


236.  A  spherical  body  will  roll  down  a  slope,  because  thu  centre 
of  gravity  is  not  supported.* 

237  Bodies,  consisting  of  but  one  kind  of  substance,  as  wood, 
stone  or  lead,  and  whose  densities  are  consequently  uniform,  will 
«it;inrl  more  firmly  than  bodies  composed  of  a  variety  of  substances, 
of  different  densities,  because  the  centre  of  gravity  in  such  cases 
more  nearly  corresponds  with  the  centre  of  magnitude. 

238.  When  a  body  is  composed  of  different  materials,  it  will 
stand  most  firmly  when  the  parts  whose  specific  gravity  is  tho 
greatest  are  placed  nearest  to  the  base. 


239.  The  broader  the  base  and  the  nearer 


When  will  a 
body   stand 

most  firmly  ?    the  centre  of  gravity  to  the  ground,  the  more 
firmly  a  body  will  stand. 

240.     For  this  reason,  high   carriages  are  more  dangerous  than 
low  ones. 
241      A  pyramid  also,  for  the  same  reason,  is  the  firmest  of  all 

Fig.  22. 


structures,  because  it  lias  a  broad  base,  and  but  little  elevation. 


vben  quite  young,  are  taught  to  practise  the  same  art.  By  means  of  these 
odd  additions  to  the  length  of  the  leg,  their  feet  are  kept  out  of  the  water 
01  the  heated 'sand,  and  they  are  also  enabled  to  see  their  sheep  at  a  greater 
distance.  They  use  these  stilts  with  great  skill  and  care,  and  run,  jump, 
and  even  dance  on  them  with  great  ease. 

*•  A  cylinder  can  be  made  to  roll  up  a  slope  by  plugging  one  side  of  it 
with  lead  ;  the  body  being  no  longer  of  a  uniform  density,  the  centre  of 
gra  vity  is  removed  from  the  middle  of  the  body  to  some  point  in  the  le*d, 
as  t  hat  substance  is  much  heavier  than  wood.  Now,  in  order  that  the  cyl- 
iud  <r  may  roll  down  the  plane,  as  it  is  here  situated,  the  centre  of  gravity 
mm  t  rise,  which  is  impossible  ;  the  centre  of  gravity  must  always  descend 
in  n  loving,  and  will  descend  by  the  nearest  and  readiest  means,  which  will 
be  by  forcing  the  cylinder  up  the  slope  until  the  centre  of  gravity  is  sup- 
ported, and  then  it  stops. 

A  body  also  in  the  shape  of  two  cones  united  at  their  bases  can  be  made  ta 
roll  up  an  inclined  plane  formed  by  two  bars  with  their  lower  ends  inclined 
towards  each  other.  This  is  illustrated  by  a  simple  contrivance  in  school 
apparatus,  and  the  fact  illustrated  is  called  "  the  mechanical  paradox." 


244.  A  person  can  carry  two  pails  of  water  more 
easily  than  one,  because  the  pails  balance  each 


242      A  cone    laa   also   the  same  stability  ;   but,  mathematically 
considered,  a  cone  is  a  pyramid  with  an  infinite  number  of  sides. 

243.     Bodies  that  have  a  narrow  base  are  easily  overset,  because 
if  they  are  but  slightly  inclined,  the  line  of  direction  will  fall  out 
side  of  the  base,  and  consequently  their  centre  of  gravity  will  not  Un- 
supported. 
Why   can    a 
person   carry 
two  pails  of 

water  more       other,  and  the  centre  of  gravity  remains  supported 

easily  than         ,  „  . 

Oflf ,1  by  the  teet.     ±Jut  a  single  pail  throws  the  centre 

of  gravity  on  one  side,  and  renders  it  more  difficult  to  support 

the  body. 

WTiere   is  the       245.    COMMON  CENTRE  OF    GRAVITY    OF    TWO 

centre  of  grav-  BODIES.— When  two  bodies  are  connected,  they 
tty  of  two  * 

bodies  connect-  are  to  be  considered  as 'forming  but  one  body,  and 
ed  together  ?  bave  but  one  ceiltre  of  gravity.  If  the  two  bodies 
be  of  equal  weight,  the  centre  of  gravity  will  be  in  the  middle 
of  the  line  which  unites  them.  But,  if  one  be  heavier  than  the 
oiher,  the  centre  of  gravity  will  be  as  much  nearer  to  the  heavier 
one  as  the  heavier  exceeds  the  light  one  in  weight. 

Figures  23          ^^'     ^*S*  ^  represents  a 
24,  and  25.     bar  with  an  equal  weight  fast- 
ened at  each  end ;  the  centre  of  gravity  is 
at  A,  the  middle  of  the  bar,  and  whatever  supports  this  centre 
will  support  both  the  bodies  and  the  pole. 

247.  Fig.  24  represents  a  bar  with  an 
unequal  weight  at  each  end.     The  centre  of 
gravity  is  at  C,  nearer  to  the  larger  body. 

248.  Fig.  25  represents  a  bar  with  un- 
equal weights  at  each  ^nd,  but  the  larger 
weight  exceeds  the  less  in  such  a  degree 
that  the  centre  of  gravity  is  within  the 
larger  body  at  C.^ 

There  are  no  laws  connected  with  the  subject  of  Natural  Science  s«j 
grand  and  stupendous  as  the  laws  of  attraction.  Long  before  the  sublime 
fiat,  "  L*  tt-erf  if  lijflu  "  was  uttered,  thr  Creator's  voice  was  heard  amid 


Fig.  23. 
A 


w 


TO  NATURAL    PHILOSOPHY. 

W/tat  things      249.  THE   MECHANICAL    POWERS.      There 

in  Mechanics 

require    dis-    are  five  things  in  mechanics  which  require  a 

iinct  consid-    Distinct  consideration,  namely  : 
eration  ?  J 

First,  the  power  that  acts. 

Secondly,  the  resistance  which  is  to  be  overcome  by  the 
power. 

Thirdly,  the  centre  of  motion,  or,  as  it  ^s  sometimes 
called,  the  fulcrum.* 

Fourthly,  the  respective  velocities  of  the  power  and  the 
resistance;  and, 

the  expanse  of  universal  emptiness,  calling  matter  into  existence,  and  sub 
jecting  it  to  these  laws.  Obedient  to  the  voice  of  its  Creator,  matter  sprang 
from  "  primeval  nothingness,  "  and,  in  atomic  embryos,  prepared  to  cluster 
into  social  unions.  Spread  abroad  in  the  unbounded  fields  of  space,  each 
particle  felt  that  it  was  "  not  good  to  be  alone.  "  Invested  with  the  social 
power,  it  aought  companionship.  The  attractive  power,  thus  doubled  by  the 
union,  compelled  the  surrounding  particles  to  join  iu  close  embrace,  and 
thus  were  worlds  created.  Launched  into  regions  of  unbound  space,  the 
new-created  worlds  found  that  their  union  was  but  a  part  of  a  great  social 
system  of  law  and  order.  Their  bounds  were  set.  A  central  point  controls 
the  Universe,  and  in  harmonious  revolution  around  this  central  point  for 
ages  have  they  rolled.  Nor  can  one  lawless  particle  escape.  The  sleepless 
eye  of  Nature's  law,  vicegerent  of  its  God,  securely  binds  them  all 
"  Could  hut  one  small,  rebellious  atom  stray, 
Nature  itself  would  hasten  to  decay." 

With  this  sublime  view  of  Creation,  how  can  we  escape  the  conclusion 
that  the  very  existence  of  a  law  necessarily  implies  a  Law-giver,  and  that 
Law-giver  must  be  the  Creator  1  Shall  we  not  then  say,  with  the  Psalmist, 
"  It  is  the  FOOL,  who  hath  said  in  his  heart  that  there  is  no  God  "  1 

Who,  then,  will  not  see  and  admire  the  beautiful  language  of  Mr.  Alison, 
while  his  heart  burns  with  the  rapture  and  gratitude  which  the  sentiments 
are  so  well  fitted  to  kindle  : 

"  When,  in  the  youth  of  Moses,  { the  Lord  appeared  to  him  in  Horeb,'  a 
/oice  was  heard,  saying,  '  Draw  nigh  hither,  and  put  off  thy  shoes  from  off 
thy  feet,  for  the  place  where  thou  standest  is  holy  ground.'  It  is  with  such 
a  reverential  awe  that  every  great  or  elevated  mind  will  approach  to  the 
study  of  nature,  and  with  such  feelings  of  adoration  and  gratitude  that  he 
will  receive  the  illumination  that  gradually  opens  upon  his  soul." 

"  It  is  not  the  lifeless  mass  of  master,  he  will  then  feel,  that  he  is  exam- 
ining; it  is  the  mighty  machine  of  Eternal  Wisdom,  —  the  workmanship  of 
Him  *  in  whom  everything  lives,  and  moves,  and  has  its  being.'  Under 
ai  aspect  of  this  kind,  it  is  impossil  le  to  pursue  knowledge  without  mingling 
with  it  the  most  elevated  sentimen.s  of  devotion  ;  —  it  is  impossible  to  per- 
ceive the  laws  of  nature  without  perceiving,  at  the  same  time,  the  presenoa 
&nd  the  providence  of  the  Law -giver  :  —  and  thus  it  is  that,  in  every  age, 
the  evidences.of  religion  have  advanced  with  the  progress  of  true  philosophy; 
and  that  SCIENCE,  IN  ERECTING  A  MONUMENT  TO  HEREBLF,  HAS,  AT  TUB  s 

ERECTED  AN  ALTAR  TO  THE  DEITY." 

*  The  word/tt/ciu/Aj  JHCUUS  a  prop,  or  support 


THE   MECHANICAL   TOWERS.  71 

Fifthly,  the  instruments  employed  in  the  construction 
of  the  machine.  (See  Appendix,  1389-1400.) 

250.  (1.)  The  power  that  acts  is  the  muscular  strength  of  men 
or  animals,  the  \v  eight  and  momentum  of  solid  bodies,  the  elastic 
force  of  steam,  springs,  the  pressure  of  the  air,  and  the  weight  of 
water,  &c. 

(2.)  The  resistance  to  be  overcome  is  the  attraction  of  gravity 
or  of  cohesion,  the  inertness  of  matter,  friction,  &<\ 

(3.)  The  centre  of  motion,  or  the  fulcrum,  is  the  point  about 
which  all  the  parts  of  the  body  move. 

(4.)  The  velocity  is  the  rapidity  with  which  an  effect  is  pro- 
duced. 

(5.)  The  instruments  are  the  mechanical  powers  which  enter 
into  the  construction  of  the  machine. 

251.  The  powers  which  enter  into  the  construc- 
What  are  .          _  ..  vr   *»•»*•*•    i 

the  Me-    struction  of  a  machine  are  called  the  Mechanical 

r.hanical    Powers.     They  are  contrivances  designed   to  in- 

Powers?  ./  . 

crease  or  to  dimmish  force,  or  to  alter  its  direction. 

What  is        252.  All  the  Mechanical  Powers  are  constructed 
dammta    on  tne  principle  that  what  is  gained  in  power  is 

principle    lost  in  time.     This  is  the  fundamental  law  of 
of     Me-     ,r     , 
chanics?    Mechanics. 

253.  If  1  Ib.  is  required  to  overcome  the  resistance  of  2  lbsN 
the  1  Ib.  must  move  over  two  feet  in  the  same  time  that  the 
resistance  takes  to  move  over  one.  Hence  the  resistance  will  move 
only  half  as  fast  as  the  power  ;  or,  in  other  words,  the  resistance 
requires  double  the  time  required  by  the  power  to  move  over  a  given 
space. 

Explain       254.  Fig.  26  illustrates  the  principle  as  applied  to  the 

l&' "  '    lever.     W  represents  the  weight,  Fig. 

F  the  fulcrum,  P  the  power,  and  the  bar 
W  F  P  the  lever.  To  raise  the  weight  W 
to  w,  the  power  P  must  descend  to  p.  But, 
as  the  radius  of  the  circle  in  which  the 
power  P  moves  is  double  that  of  the  radius 
•jf  the  circle  in  which  the  weight  W  moves, 


\  I  NATURAL    PHILOSOPHY. 

tkj  arc  P  p  is  double  the  arc  VV  w  ;  or,  in  other  words,  the  dis 
ta^ice  P  p  is  double  the  distance  of  W  w.  Now,  as  these  dis* 
fauces  are  traversed  in  the  same  time  by  the  power  and  the 
weight  respectively,  it  follows  that  the  velocity  of  the  power 
must  be  double  the  velocity  of  the  weight;  that  is,  the  power 
niuat  move  at  the  rate  of  two  feet  in  a  second,  in  order  to  move 
th*  weight  one  foot  in  the  same  time. 

This  principle  applies  not  only  to  the  lever,  but  to  all  the 
Mechanical  Powers,  and  to  all  machines  constructed  on  me 
chanical  principles. 

How  many  Me-       255.  There  are  six  Mechanical  Powers  :* 

±tmf^T the  Lever> the  Wkeel  and  Axle' the  Pulley> 

their  names  f      the  Inclined  Plane,  the  Wedge  and  the  Screw. 

All  instruments  and  machines  are  constructed  on  the  principle  of  one 
or  more  of  the  Mechanical  Powers. 

All  the  Mechanical  Powers  may  be  reduced  to  three  classes,  namely  •• 
1st,  a  body  revolving  on  an  axis  ;  2d,  a  flexible  cord  ;  and,  3d',  an  inclined 
surface,  smooth  and  hard.  To  the  first  belongs  the  lever,  and  the  whee] 
and  axle  ;  to  the  second,  the  pulley  ;  to  the  third,  the  inclined  plane,  the 
wedge  and  the  screw. 

What  is  the  256.  The  Lever  is  an  inflexible  bar,  mova- 
Lever ,  and  how  ,,  c  , 

is  it  used  ?  "le  on  a  fulcrum  or  prop. 

It  is  used  by  making  one  part  to  rest  on  a  fulcrum,  applying  the 
power  to  bear  on  another  part,  while  a  third  part  of  the  lever 
cpposes  its  motion  to  the  resistance  which  is  to  be  overcome. 

257.  In  every  lever,  therefore,  whatever  be  its  form,  there  are 
three  things  to  be  distinctly  considered,  namely  :  the  position  of  the 
fulcrum,  of  the  power,  and  of  the  weight,  respectively.  It  is  the 
position  of  these  which  makes  the  distinction  between  the  different 
kinds  of  levers. 

How  many  kinds        258.   There  are  three   kinds   of   levers, 

\)j      levers      are 

here  J  called  the  first,  second  and  third,  according 

to  the  respective  position  of  the  fulcrum,  the  power,  and 
the' weight. 

These  may  be  represented  thus  : 
Power,  Fulcrum,  Weight, 

Power,  Weight,  Fulcrum, 

Weight,  Power,  Fulcrum 

*  More  properly  called  simple  machines. 


THE    MECHANICAL    POWERS. 


the 
position  of  the 

voider,  the 


the  fiiicrum, 
respectively,  hi 
the  tnree  kinds 


Describe  a  ~ever 
oj  the  first  kind 
by  figure  27, 
and  tell  the  ad- 


fig.  27 


That  is,  (1.)  The  poyer^  is  at  one  end,  the 
weight  at  the  other,  and  the  fulcrum  between  them. 

(2.)  Power  at  one  end,  the  fulcrum  at  tho 
other,  andthe  weight  between  them. 

(3)  Th^weight  is  at  one  end,  the  fulcrum  at 
the  other,  and  the  power  between  them. 

259.  In  a  lever  of  the  first  kind  the  fulcrum 
is  placed  between  the  power  and  the  weight. 

Fig.  27  represents  a  lever  of  the  first  kind 

vantage  gained    resting  on  the  fulcrJn 
ljy  it.  T,  ,  ,  , 

r ,    and  movable   upon 

it.  W  is  the  weight  to  be  moved,  and 
P  is  the  power  which  moves  it.  The 
advantage  gained  in  raising  a  weight, 
by  the  use  of  this  kind  of  lever,  is  in 
proportion  as  the  distance  of  the  power  from  the  fulcrum  exceeds 
that  of  the  weight  from  the  fulcrum.  Thus,  in  this  figure,  if 
the  distance  between  P  and  F  be.  double  that  between  W  and 
F,  then  a  man,  by  the  exertion  of  a  force  of  100  pounds  with 
the  lever,  can  move  a  weight  of  200  pounds.  From  this  it  fol- 
lows that  tine,  nearer  the  power  is  applied  to  the  end  of  the  lever  > 
the  greater  is  the  advantage  gained.  Thus,  a  greater  weight 
can  be  moved  by  the  same  power  when  applied  at  13  than  when 
it  is  exerted  at  P. 


On  what  prin- 
ciple is  the  com- 
mon steelyard 
Constructed? 
•  Describe  the 
steelyard. 


260.  The  common  steelyard,  an  instrument  for 
weighing  articles,  is  constructed  on  the  principle 
of  the  lever  of  the  first  kind.  It  consists  of  a 
rod  or  bar,  marked  with  notches  to  designate  the 
pounds  and  ounces,  and  a  weight,  which  is  inova- 


*  It  is  to  be  understood,  in  the  consideration  of  all  instruments  and  ma- 
chines, that  some  effect  is  to  be  produced  by  some  power.  The  names 
fM>wer  and  weight  are  not  always  to  be  taken  literally.  They  are  terms 
usefl  to  express  the  cause  and  the  effect.  Thus,  in  the  movement  of  a  clock, 
*;he  weight  is  the  cause,  the  movement  of  the  hands  ib  the  effect.  The 
cilice  of  motion,  whether  it  be  a  weight  or  a  resistance,  is  technically  called 
the  power  ;  the  effect,  whether  it  be  the  raising  of  a  weight,  the  overcoming 
of  resistance  or  of  cohesion,  the  separation  of  the  parts  of  a  body,  couiprea 
»u»u  tir  expansion,  is  technically  called  the 


74 


NATURAL    PHILOSOPHY. 


ole  along  the  notches.  The  bar  is  furnished  with  throe  hoc**, 
on  the  longest  of  which  the  article  to  be  weighed  is  always  to  bf 
fang.  The  other  two  hooka  serve  for  the  handle  of  the  instru 


Fig.  28. 


ment  when  in  use.  The  pivot  of  each  of  these  two  hooks  serves 
for  the  fulcrum. 

261.  When  suspended  by  the  hook  C,  as  in  Fig. 

are  'th^tfoee  28>  {i  is  manifest  that  a  Pound  weight  at  E  wil1 
hooks  in  the  balance  as  many  pounds  at  W  as  the  distance  be- 
stedyard?  tween  tne  pjvot  Of  j)  an(j  tne  pivot  Of  Q  Js  con. 

tained  in  the  space  between  the  pivot  of  C  and  the  ring  front 
which  E  is  suspended. 

The  same  instrument  may  be  used  to  weigh  heavy  articles 
by  using  the  middle  hook  for  a  handle,  where,  as  will  be  seen 
in  Fig.  29,  the  space  between  the  pivot  of  F  (which  in  this 
case  is  the  fulcrum)  and  the  pivot  of  D  (from  which  the  weight 
is  suspended)  being  lessened,  is  contained  a  greater  number  of 
times  in  the  distance  between  the  fulcrum  and  the  notches  on 
the  bar.  The  steelyard  is  furnished  with  two  sets  of  notches  on 
apposite  sides  of  the  bar.  An  equilibrium  *  will  always  be 


Of  Equilibrium.  —  In  the  calculations  of  the  powers  of  all  machines  it  if 


THE    MECHANICAL    POWERS. 


produvA,<3  when  the  product  of  the  weights  on  the  opposite  sides 
of  the  fulcrum  into  their  respective  distances  from  it  are 
to  one  another. 

Fig.  29. 


A  balance,  or  pair  of  scales,  is  a  lever  of  the  first  kind,  with 
equal  arms.  Steelyards,  scissors,  pincers,  snuffers,  and  a  poker 
used  for  stirring  the  fire,  are  all  levers  of  the  first  kind.  The 
longer  the  handles  of  scissors,  pincers,  &c.,  and  the  shorter  the 
points,  the  more  easily  are  they  used.  (See  Appendix,  par.  1415.) 

262.  The  lever  is  made  in  a  great  variety  of  forms  and  of  many 
different  materials,  and  is  much  used  in  almost  every  kind  of 
mechanical  operation.  Sometimes  it  is  detached  from  the  fulcrum 

necessary  to  have  clearly  in  inind  the  difference  between  action  and  equi- 
librium. 

By  equilibrium  is  meant  an  equality  of  forces  ;  as,  when  one  force  is 
opposed  by  another  force,  if  their  respective  momenta  are  equal,  an  equi- 
librium is  produced,  and  the  forces  merely  counterbalance  each  other.  To 
produce  any  action,  there  must  be  inequality  in  the  condition  of  one  of  the 
forces.  Thus,  a  power  of  one  pound  on  the  longer  arm  of  a  lever  will  bal 
ance  a  weight  of  two  pounds  on  the  shorter  arm,  if  the  distance  of  tlie 
power  from  the  fulcrum  be  exactly  double  the  distance  of  the  weight  from 
the  fulcrum  ;  and  the  reason  why  they  exactly  balance  is,  because  their 
momenta  are  equal.  No  motion  can  be  produced  or  destroyed  without  a 
difference  between  the  force  and  the  resistance.  In  calculating  the  me- 
chanical advantage  of  any  machine,  therefore,  the  condition  of  equilibrium 
must  first  be  duly  considered.  After  an  equilibrium  is  produced,  whatevei 
is  added  upon  the  one  side  or  taken  away  on  fie  other  destroys  the  equi- 
librium, and  causes  the  machine  to  move 


76  NATURAL    PHILOSOPHY. 

nut  most  generally  the  fulcrum  is  a  pin  or  rivet  by  which  the 

is  permanently  connected  with  the  frame-work  of  other  parts  of  the 

machinery. 

263.  When  two  weights  are  equal,  and  the  fulcrum  is  placed 
exactly  in  the  centre  of  the  lever  between  them,  they  wrill  mutually 
balance  each  other ;  or,  in  other  words,  the  centre  of  gravity  being 
supported,  neither  of  the  weights  will  sink.  This  is  the  principle 
of  the  common  scale  for  weighing. 

HM  is  power  264.  To  gain  power  by  the  use  of  the 
usTofthe  l  ^ver,  the  fulcrum  must  be  placed  near  the 
lever?  weight  to  be  moved,  and  the  power  at  the 

greater  distance  from  it.  The  force  of  the  lever,  there- 
fore, depends  on  its  length,  together  with  the  power 
applied,  and  the  distance  of  the  weight  from,  the  ful- 
crum.* 

Wha'isa  2t>5-    A   Com-  ^  ^ 

Compound     pound  Lever,  rep- 
resented in  Fig. 

30,  consists  of  several  levers, 

so  arranged  that  the  shorter 

arm  of  one  may  act  on  the  longer  arm  of  the  other.  Great 
power  is  obtained  in  this  way,  but  its  exercise  is  limited  to  a 
very  small  space. 

Describe  the  266.  In  a  lever  of  the  second  kind,  the  ful- 

lever  of  the  sec-  crum  [a  at  one  enci  the  power  at  the  other,  and 
ond  fand,with 

Fig.  31.  the  weight  between  them. 

(1.)  Let  Fig.  31  represent  a  lever  of  the  second  kind.  F  is 
the  fulcrum,  P  the  power,  and  W  the  weight.  Fig  31t 

The  advantage  gained  by  a  lever  of  this  kind  is     J/ 

in  proportion  as  the  distance  of  the  power  from  pu,,,,,,,,, , ,„..„£ 

the  fulcrum  exceeds  that  of  the  weight  from  the 

fulcrum.     Thus,  in  this  figure.,  if  the  distance  w 

*  This  being  the  case,  it  is  evident  that  the  shape  of  the  lever  will  not 
influence  its  power,  whether  it  be  straight  or  bent.  The  direct  distanr?  between 
the  fulcrum  and  the  weight,  compared  with  the  same  distance  between  the 
fulcrum  and  the  power,  being  the  only  measure  of  the  mechanical  advantage 
*hieh  it  afford* 


THE    MECHANICAL    POWEKS.  77 

from  P  to  F  is  four  times  the  distance  from  W  to  F,  then  & 
power  of  one  pound  at  P  will  balance  a  weight  of  four  pounds 
at  W. 

(2.)  On  the  principle  of  this  kind  oflever,  two  persons,  carrying 
a  heavy  burden  suspended  on  a  bar,  may  be  made  to  bear  unequal 
portions  of  it,  by  pi  icing  it  nearer  to  the  one  than  the  other. 

267.  Two  horses  also,  may  be  made  to  draw  unequal  portuns  of 
a  load,  by  dividing  the  bar  attached  to  the  carriage  in  euch  a 
manner  chat  the  weaker  horse  may  draw  upon  the  longer  end  of  it.. 

208.  Oars,  rudders  of 
ships,  doors  turning  on 
hinges,  and  cutting-knives 
which  are  fixed  at  one  end, 
are  constructed  upon  the 
principle  of  levers  of  the 
second  kind.* 

Describe  the  269.  In  a  lever  of  the  third  kind  the  fulcrum 

lthird°kindl  is  at  One  6nd'  the  we  Sut  at  the  otlier'  and  the 
Fig.  33.  power  is  applied  between  them. 

In  levers  of  this  kind  the  power  must  always  exceed  the 
weight  in  the  same  proportion  as  the  distance  of  the  weight 
irom  t  he  fulcrum  exceeds  that  of  the  power  from  the  fulcrum 

In  Fig.  33  F  is  the  fulcrum,  W  the  weight,  Fig.  33. 
and  P  the  power  between  the  fulcrum  and  the 
weight ;  and  the  power  must  exceed  the  weight 
in  the  same  proportion  that  the  distance  between 
W  and  F  ex2eeds  the  distance  between  P 
and  P. 

270.  A  ladder,  which  is  to  be  raised  by  the  strength  of  a  man's 
arms,  represents  a  lever  of  this  kind,  where  the  fulcrum  is  that 
end  which  is  fixed  against  the  wall ;  the  weight  may  be  consid- 
ered as  at  the  top  part  of  the  ladder,  and  the  power  is  the  strength 
applied  in  raising  it. 

271.  The  bones  of  a  man's  arm,  and  most  of  the  movable  bones 
of  animals,  are  levers  of  the  third  kind.     But  the  loss  of  power  in 
limbs  of  animals  is  compensated  by  the  beauty  and  compactness  of 

*  It  is  on  the  same  principle  that,  in  raising  a  window,  the  hand  should 
be  applied  to  the  middle  of -the  sash,  as  it  will  then  be  easify  raised; 
whereas,  if  the  hand  be  applied  nearer  to  one  side  than  the  other,  the 
centre  of  gravity  being  unsupported,  will  cause  the  further  side  to  bear 
against  the  frame,  and  obstruct  its  free  motion. 


78  NATURAL    PHILOSOPHY. 

the  hinbs,  as  well  as  the  increased  velocity  of  their  motion.  Tna 
wheels  in  clock  and  watch  work,  and  in  various  kirds  of  machinery, 
may  be  considered  as  levers  of  this  kind,  when  the  power  that 
moves  them  acts  on  the  pinion,  near  the  centre  of  motion,  and  the 
resistance  to  be  overcome  acts  on  the  teeth  at  the  circumference. 
But  here  the  advantage  gained  is  the  change  of  slow  into  rapid 
motion 

272.  PRACTICAL  EXAMPLES  OF  LEVERAGE. 
Questions  for  Solution 

(1.)  Suppose  a  lever,  6  feet  in  length,  to  be  applied  to  raise  a  weight  of  50  pounds, 
with  a  power  of  only  1  pound,  where  must  the  fulcrum  be  placed  ?  An*.  1.41  in.  + 

(2.)  If  a  man  wishes  to  move  a  stone  weighing  a  ton  with  a  crow-bar 
6  feet  in  length,  he  himself  being  able,  with  his  natural  strength,  to  move  u 
weight  of  100  pounds  only,  what  must  be  the  greatest  distance  of  the  ful- 
crum from  the  stone  1  Ana.  8.42  in.  -f- 

(3.)  If  the  distance  of  the  power  from  the  fulcrum  be  eighteen  timei 
greater  than  the  distance  of  the  weight  from  .he  fulcrum,  what^power  wiauld 
be  required  to  lift  a  weight  of  1000  pounds  1  Ann.  55.55  Ib.  + 

(4.)  If  the  distance-  of  the  weight  from  the  fulcrum  be  only  a  tenth  of 
the  distance  of  the  power  from  the  fulcrum,  what  weight  can  be  raised  by  a 
power  of  170  pounds  1  Ans.  1700  Ib. 

(5.)  In  a  pair  of  steelyards  the  distance  between  the  hook  on  which  the 
weight  is  hung  and  the  hook  by  which  the  instrument  is  suspended  is  2 
inches  ;  the  length  of  the  steelyards  is  30  inches.  How  great  a  weight  may 
be  suspended  on  the  hook  to  balance  a  weight  of  2  pounds  at  the  extremity 
of  the  longer  arm  1  Ans.  28  Ib. 

(G.)  Archimedes  boasted  that,  if  he  could  have  a  place  to  stand  upon,  he 
oould  move  the  whole  earth.  Now,  suppose  that  he  had  a  fulcrum  with  a 
lever,  and  that  his  weight,  compared  with  that  of  the  earth,  was  as  1  to 
270  millions.  Suppose,  also,  that  the  fulcrum  were  a  thousand  miles  from 
the  earth ;  wh;it  mu»t  be  his  distance  from  the  fulcrum  ? 

Ans.  270,000,000,000  mi. 

(7.)  Which  will  cut  the  more  easily,  a  pair  of  scissors  9  inches  long, 
tvith  ihe  rivet  5  inches  from  the  points,  or  a  pair  of  scissors  6  inches  long, 
with  the  rivet  4  inches  from  the  points  1  Ans.  The  first 

(8.)  Two  persons,  of  unequal  strength,  carry  a  weight  of  200  pounds 
suspended  from  a  pole  10  feet  long.  One  of  them  can  carry  only  75  pounds, 
the  other  must  carry  the  rest  of  the  weight.  How  far  from  the  end  of  the 
pole  must  the  weight  be  suspended  7  Ans.  8.75ft. 

(9.)  How  must  the  whiffle-tree  *  of  a  carriage  be  attached,  that  one  horse 
may  draw  but  3  cwt.  of  the  load,  while  the  other  draws  5  cwt.  1  Ans.  At  j. 

(10.)  On  the  end  of  a  steelyard,  3  feet  long,  hangs  a  weight  of  4  pounds, 
suppose  the  hook,  to  which  articles  to  be  weighed  are  attached,  to  be  at 
the  extremity  of  the  other  end,  at  the  distance  of  4  inches  from  the  hook 
by  which  the  steelyards  are  held  up.  How  great  a  weight  can  be  estimated 
by  the  steelyard  1  Ans.  32  Ib. 

What  is  the  273.  THE  WHEEL  AND  AXLE.  —  The 
tek?  '  Wheel  and  Axle  consists  of  a  cylinder  with  a 

wheel  attached,  both  revolving  around  the  same  axis  of  motion. 

*  The  whiffle-tree   is   gererally  attached   to  a   carriage   by  a  huok  or 
<oather  band  in  the  centre,  sc  that  the  draft  shall  be  equal  on  both 
Hie  hook  or  leather  band  thus  becomes  a  fulcrum. 


THli    MECHANICAL    TOWERS. 


are  the          274.  The  weight   is   supported  by  a  rope  or 

power  an    t  e    ^^  wound  around  the  cylinder;  the  power  is 

weight  applied       •  J  * 

to  the  wheel       applied  to  another  rope  or  chain  wound  around 

and  axle?          ^he   circumference  of   the  cylinder.     Sometimes 
projecting  spokes  from  the  wheel  supply  the  place  of  the  chain,* 

275.  The  place  of  the  cylinder  is  sometimes  supplied  by  a  small 
wheel. 


axle  by  Fig. 


axle,   though    made 
be   understood   b 


Fig.  34. 


n 


Explain  the  276.  The   wheel   and 

man?   forms'   w 
specting     Figs. 

34  and  35.     In 

Fig.  34  P  represents  the  larger 
wheel,  where  the  power  is  ap- 
plied ;  C  the  smaller  wheel,  or 
cylinder,  which  is  the  axle  ; 
and  W  the  weight  to  be  raised. 

What  is  the  The  advantage 
advantage  imd  is  Jn 

gained  by  the 

use  of  the  wheel  proportion  as 
znd  axle  ?  the  circumfer- 

ence of  the  wheel  is  greater 
than  that  of  the  axle.     That 
is,  if  the  circumference  of  the  wheel  be  six  times  the  circum- 
ference of  the  axle,  then  a  power  of  one  pound  applied  at  the 
wheel  will  balance  a  power  of  six  pounds  on  the  axle. 
How  does  the         277.     Some-  «*•  «*. 

times  the  axle 
constructed 


wheel  and  axle 
described  in 
Fig.  35  differ 
from  that  de- 
scribed in  Fig. 
34? 


is 


with  a  winch  or 
handle,  as  in 
Fig.  35,  and 
sometimes  the  wheel  has  pro- 
jecting spokes,  as  in  Fig.  34. 


*    A  cylinder  is  a  long  circular  body  of  uniform  dimmer,  witb  extremities 
fonniug  equal  and  parallel  circles 


80  NATURAL    PHILOSOPHY. 


On  what    ri  ^^'  ^6  Prmcip^e  uPon  which  the        3el  and 

*ipl.e  is  the         axle  is  constructed  is  the  same  with  tWtft  of  the 


jheel  and  axh  Other  Mechanical  Powers,  the  want  of  powei 
constructed  /  ,  .  ,11  i  •  T  •  T 

being  compensated    by  velocity.     It  is   evident 

(from  the  Figs.  34  and  85)  that  the  velocity  of  the  circum- 
ference of  the  wheel  is  as  much  greater  tha*n  that  of  the  axle  as 
it  is  further  from  the  centre  of  motion  ;  for  the  wheel  describes 
a  great  circle  in  the  same  time  that  the  axle  describes  a  small 
one  ;  therefore  the  power  is  increased  in  the  same  proportion  as 
the  circumference  of  the  wheel  is  greater  than  that  of  the  axle. 
If  the  velocity  of  the  wheel  be  twelve  times  greater  than  that 
of  the  axle,  a  power  of  one  pound  on  the  wheel  will  support  a 

weight  of  twelve  pounds  on  the  axle. 

« 

279.  The  wheel  and  axle  are   sometimes  called  "  the  continuous 
lever,"  the  diameter  of  the  wheel  representing  the  longer  arm,  the 
diameter  of  the   axle    representing  the  shorter  arm,   the  fulcrum 
being  at  the  common  centre. 

280.  The  capstan,*  on  board  of  ships  and  other  vessels,  is  con 
structed  on  the  principle  of  the  wheel  and  axle.     It  consists  of  an 
axle  placed  uprightly,  with  a  head  or  drum,  pierced  with  holes  for 
the  lever,  or  levers,  which  supply  the  place  of  the  wheel. 

281.  VVindmills,  lathes,  the  common  windlass,  used  for  drawing 
water  from  wells,  and  the  large  wheels  in  mills,  are  all  constructed 
on  the  principle  of  the  wheel  and  axle. 

282.  Wheels  are  a  very  essential  part  to  most  machines.     They 
are  applied  in  different  ways,  but,  when  affixed  to  the  axle,  their 
mechanical  power  is  always  in  the  same  proportion  ;  that  is,  as 
the  circumference  of  the  wheel  exceeds  that  of  the  axle,  so  much 
will  the  power  be  increased.     Therefore,  the  larger  the  wheel,  and 
the  -smaller  the  axle,  the  greater  will  be  the  power  obtained. 

'  283.  CRANKS.  —  Cranks    are    sometimes    con- 

Cranks  and  nected  with  the  axle  of  a  wheel,  either  to  give  or 
how  are  they  to  receive  its  motion.  They  are 
made  by  bending  the  axle  in  -such  a 
manner  as  to  form  four  right  angles  facing  in  dif- 
ferent directions,  as  is  represented  in  Fig.  36. 
They  are,  in  fact,  nothing  more  than  a  double  winch. 

*  The  difference  between  a  capstan  and  a  windlass  lies  only  in  the 
position  of  the  wheel.  If  the  wheel  turn  horizontally,  it  is  called  a  capstan; 
if  vertically,  a  windlass. 


T1IH-    MECHANICAL    Pi/WKRS.  81 

284.  A  rod  connects  the  crank  with  other  parts  of  the  machinery 
either  to  communicate  motion  to  or  from  a  wheel.     When  the  rod 
which  communicates  the  motion  stands  perpendicular  to  the  crank, 
which  is  the  case  twice  during  each  revolution,  it  is  at  what  is 
commonly  called  the  dead  point,  and  the  crank  loses  all  its  power. 
But,  when  the  rod  stands  obliquely  to  the  crank,  the  craak  is  then 
effective,  and  turns  or  is  turned  by  the  wheel. 

285.  Cranks  are  used  in  the  common  foot-lathe  to  turn  the  wheel 
They  are  also  common  in  other  machinery,  and  are  very  convenient 
for  changing  rectilinear  to  circular  motion,  or  circular  to  rectilinear 

286.  When  they  communicate  motion  to  the  wheel  they  operate 
like  the  shorter  arm  of  a  lever ;  and,  on  the  contrary,  when  they 
communicate  the  motion  from  the  wheel  they  act  like  the  longer 
arm. 

T*™  *       n?          287.    FLY-WHEELS   are   heavy   rims   of  metal 
W  fiat  are  t  Ly- 

whee/s,  and  secured  by  light  spokes  to  an  axle.  They  are 
what  is  their  use(j  to  accumulate  power,  and  distribute  it 
equally  among  all  the  parts  of  a  machine.  They 
are  caused  to  revolve  by  a  force  applied  to  the  axle,  and,  when 
once  set  in  motion,  continue  by  their  inertia  to  move  for  a  long 
time.  As  their  motion  is  steady,  and  without  sudden  jerks, 
they  serve  to  steady  the  power,  and  cause  a  machine  to  work 
with  regularity. 

288.  Fly-wheels  are  particularly  useful  in  connexion  with  cranks, 
especially  when  at  the  dead  points,  as  the  momentum  of  the  fly- 
wheel, received  from  the  cranks  when  they  acted  with  most  advan- 
tage, immediately  carries  the  crank  out  of  the  neighborhood  of  the 
lead  points,  and  enables  it  to  again  act  with  advantage. 

289.  There  are  two  ways  in  which  the  wheel  and  axle  is  sup- 
ported  namely,  first  on  pointed  pivots,  projecting  into  the  extrem- 
ities of  the  axle,*  and,  secondly,  with  the  extremities  of  the  axle 
resting  on  gudgeons.     As  by  the  former  mode  a  less  extensive  are** 
is  subjected  to  friction,  it  is  in  many  cases  to  be  preferred. 

How  many  290.    WATER-WHEELS.  —  There   are     four 

kinds  of  kindg  of  \Yater-wheels.   called,  respectively, 

*  The  terms  axle,  axis,  arbor  and  shaft,  are  synonymously  used  by 
mechanics  to  express  the  bar  or  rod  which  passes  through  the  centre  of  a 
jrheel.  The  terminations  of  a  horizontal  arbor  are  called  gudgeons,  and 
of  an  upright  one  frequently  pivots  ;  but  gudgeons  more  frequently  denote 
the  beds  on  which  the  extremities  of  the  axle  revolve,  and  pivots  are 
either  the  pointed  extremities  of  an  axle,  or  short  pins  in  the  frame  of  a 
machine  which  receive  the  extremities  of  the  axle.  The  term  axis,  in  a 
more  exact  sense,  may  mean  merely  the  kngesi  central  diameter,  or  a 
diameter  about  which  motion  takes*  place 


NATURAL   PHILOSOPHY. 


Water-wheels     the  Overshot,  the  Undershot,  the  Breast,  and 
are  there  ?         tne  Turbine.     (See  par.  1440  to  1450.) 

291.  The  Overshot  Wheel  receives  its  motion  from  the 
weight  of  the  water  flowing  in  at  the  top.  (See  par.  1441.) 
Describe  the  ^8*  ^  represents  the  Overshot  Wheel.  It  con- 


Overshot 
Wheel. 


Fig.  37 


sists  of  a  wheel  turning  on  an  axis  (not  repre- 
sented in  the  figure),  with 
compartments  called  buckets,  abed,  &c., 
at  the  circumference,  which  are  succes- 
sively filled  with  water  from  the  stream 
S.  The  weight  of  the  water  in  the  buckets 
causes  the  wheel  to  turn,  and  the  buckets, 
being  gradually  inverted,  are  emptied  as 
they  descend.  It  will  be  seen,  from  an 
inspection  of  the  figure,  that  the  buckets  in  the  descending  side 
of  the  wheel  are  always  filled,  or  partly  filled,  while  those  in 
the  opposite  or  ascending  part  are  always  empty  until*they  are 
again  presented  to  the  stream.  This  kind  of  wheel  is  the  most 
powerful  of  all  the  water-wheels. 

292.  The  Undershot  Wheel  is  a  wheel  which  is  moved  by  the 
motion  of  the  water,  receiving  its  impulse  at  the  bottom.  (See 
par.  1443.) 

Fig.  38  rep- 
resents the  Un- 
dershot Wheel. 

Instead  of  buckets  at  the  cir- 
cumference, it  is  furnished 

with    plane   surfaces,    called 

float-boards,  abed,  &c.,  which 

receive   the   impulse   of  the 

water,  and  cause  the  wheel 

to  revolve. 


Describe  the 

Undershot 

Wheel. 


Fig.  38. 


D> -scribe  the 
Bread  Wheel 


293.  The  Breast  Wheel  is  a  wheel  which  receives 
the  water  at  about  half  its  own  height,  or  at  the 


THE   MECHANICAL   POWERS. 


83 


rig 


level  of  its  own  axis.  It 
is  moved  by  the  weight 
and  acquired  force  of  the 
water. 

Fig.  39  represents  a 
Breast  Wheel.  It  is  fur- 
nished either  with  buck- 
ets or  with  float-boards, 
fitting  the  water-course,  receiving  the  weight  of  the  water  with 
its  force,  while  in  motion  it  turns  with  the  stream.  (See  Appen- 
dix, par.  1442.) 

294.  In  the  water-wheels  which  have  now  been  described,  the 
motion  is  given  to  the  circumference  of  the  larger  wheel,  either  by 
the  weight  of  the  water  or  by  its  force  when  in  motion. 

295.  All  wheels  used  in  machinery  are  connected  with  the  differ- 
ent parts  of  the  machine  by  other  parts,  called  gearing.     Sometimes 
they  are  turned  by  the  friction  of  endless  bands  or  cords,  and  some- 
times by  cogs,  teeth,   or  pinions.      When   turned  by  bands,  the 
motion  may  b*  direct  or  reversed  by  attaching  the  band  with  one  or 
two  centres  of  motion  respectively. 

296.  When  the  wheel  is  intended  to  revolve  in 
the  same  direction   with   the   one  from  which  it 
receives  its   motion,  the   band   is  attached  as  in 
Fig.  40  ;  but  when  it  is  to  revolve  in  a  contrary 
direction,  it  is  crossed  as  in  Fig.  41.     In  Fig.  40 
the  band  has  but  one  centre  of  motion ;  in  Fig.  41 
it  has  two. 

297.  Instead  of  the  friction  of  bands,  the  rough 
surfaces  of  the  wheels  themselves  are  made  to  com- 
municate their  motion.      The   wheels  and  axles  thus  rubbing  to 
gether  are  sometimes  coated  with  rough  leather,  which,  by  increas- 
ing the  friction,  prevents  their  slipping  over  one  another  without 
communicating  motion. 

298.  Figure  42  represents  suoii  a  combination  of  wheels 
the  wheel  a  is  turned  by  the  weight  S,  its  axle 

presses  against  the  circumference  of  the  wheel  b, 
causing  it  to  turn ;  and,  as  it  turns,  its  axle  rubs 
against  the  circumference  of  the  wheel  c,  which 
in  like  manner  communicates  its  motion  to  d. 
Now,  as  the  circumference  of  the  wheel  a  is  equal 
to  six  times  the  circumference  of  its  axle,  it  is 
evident  that  when  the  wheel  a  has  made  one  rev- 
olution b  will  have  performed  only  one-sixth  of  a 
revolution.  The  wheel  a  must  therefore  turn  round  six  times  tc 
cause  b  to  turn  once.  In  like  manner  b  must  perform  six  revolutions 


Fig.  40 


84 


NATURAL   PHILOSOPHY. 


to  cause  c  to  turn  once,  and  c  must  turn  as  many  times  to  cause  d  to 
revolve  once.  Hence  it  follows  that  while  d  revolves  once  on  its 
axis  c  must  revolve  six  times,  6  thirty-six  times,  and  a  two  hundred 
and  sixteen  times. 

299.  If,  on  the  contrary,  the  power  be  applied  at  F,  the  conditions 
will  all  be  reversed,  and  c  will  revolve  six  times,  b  thirty-six,  and  a 
two  hundred  and  sixteen  times.  Thus  it  appears  that  we  may 
obtain  rapid  or  slow  motion  by  the  same  combination  of  wheels. 

How  may  rapid  or  300.  To  obtain  rapid  motion,  the  power 

sljw  motion  be  ob-  .     ,             , .    -,    ,                   ,                 t  ,    . 

trine*  at  pleasure  must   be  applied  to  the  axle ;  to  obtain 

by  a  combination  of  slow  motion,  the  power  must  be  applied  to 


the  circumference  of  the  wheel. 


C 


wheels    with    their 
axles  'f 

301.  Wheels  are  sometimes  moved  by  means  of  cogs   or  teeth 
articulating  one  with  another,  on  the  circumference   of  the  wheel 
and  the  axle.     The  cogs  on  the  surface  of  the  wheels  are  generally 
called  teeth,  and  those  on  the  surface  of  the  axle  are  called  leaves. 
The  axle  itself,  when  furnished  with  leaves,  is  called  a  pinion. 

302.  Fig.   43   represents   a  connexion   of  cogged   wheels.     The 
wheel  B,  being  moved   by   a 

string  around  its  circumfer- 
ence, is  a  simple  wheel,  with- 
out teeth.  Its  axle,  being  fur- 
nished with  cogs  or  leaves,  to 
which  the  teeth  of  the  wheel 
D  are  fitted,  communicates  its 
motion  to  D,  which,  in  like 
manner,  moves  the  wheel  C. 
The  power  P  and  the  weight 
W  must  be  attached  to  the 
circumference  of  the  wheel  or 
of  the  axle,  according  as  a  slow 
or  a  rapid  motion  is  desired. 

303.  Wheels  with  teeth  or  cogs  are  of  three  kinds,  according  tf 


W 


Fig.  44 


Fig.  45. 


the  position  of  the  teeth.     When  the  teoth  are  raised  perpendicular 
to  the  axis,  they  are  called  spur  wheels  or  spur  gear.     When  the 


THE   MECHANICAL   POWEKS.  85 

teetli  are  parallel  with  the  axis,  they  are  called  crown  wheels.  When 
they  are  raised  on  a  surface  inclined  to  the  axis,  they  are  called 
bevelled  wheels.  In  Fig.  43  the  wheels  are  spur  wheels.  In  Figs.  44 
and  45  the  wheels  are  bevelled  wheels. 

304.  Different  directions  may  be  given  to  the  motion  produced 
by  wheels,  by  varying  the  position  of  their  axles,  and  causing  them 
to  revolve  in  different  planes,  as  in  Fig.  44  ;  or  by  altering  the  shape 
and  position  of  the  cogs,  as  in  Fig.  45. 

How  may  the  305.    The  power  of  toothed  wheels  may  be 

^wheds  °fbe°0te^i  estimated  bJ  substituting  the  number  of  teeth 
mated?  in  the  wheel  and  the  number  of  leaves  in  the 

pinion  for  the  diameter  or  the  circumference  of  the  wheel  and 
axle  respectively. 

306.  SUSPENSION  OF  ACTION.  —  In  the  arrangement  of  machinery, 
it  is  often  necessary  to  cut  off  the  action  of  the  moving  power  from 
some  parts,  while  the  rest  continues  in  motion.     This  is  done  by 
causing  a  toothed  wheel  to  slide  aside  in  the  direction  of  its  axis  to  an.'l 
from  the  cogs  or  leaves  into  which  it  articulates,  or,  when  the  motion 
is  communicated  by  a  band,  by  causing  the  band  to  slip  aside  from 
the  wheel  to  another  wheel,  which  revolves  freely  around  the  axle, 
without  communicating  its  motion. 

307.  Wheels  are  used  on  vehicles  to  diminish  the  friction  of  the 
road.     The  larger  the  circumference  of  the  wheel,  the  more  readily 
it  will  overcome  obstacles,  such  as  stones  or  inequalities  in  tho 
surface  of  the  road. 

308.  A  large  wheel  is  also  attended  with  two  additional  advan- 
tages ,  namely,  first,  in  passing  over  holes,  ruts  and  excavations,  a 
large  wheel  sinks  less  than  a  small  one,  and  consequently  causes  less 
jolting  and  expenditure  of  power ;  and,  secondly,  the  wear  of  large 
wheels  is  less  than  that  of  small  ones,  for,  if  we  suppose  awheel  six 
feet  in  diameter,  it  will  turn  round  but  once  while  a  wheel  three* feet 
in  diameter  will  turn  round  twice,  its  tire  will  come  twice  as  often 
to  the  ground,  and  its  spokes  will  twice  as  often  have  to  bear  the 
weight  of  the  load. 

309.  But  wheels  must  be  limited  in  size  by  two  considerations  : 
first,  the  strength  of  the  materials  ;  and  secondly,  the  centre  of  th/: 
wheel  should  never  be  higher  than  the  breast  of  the  horse,  or  other 
animal  by  which  the  vehicle  is  drawn  ;  for  otherwise  the  animal 
would  have  to  draw  obliquely  downward,  as  well  as  forward,  aiid 
thus  expend  part  of  his  strength  in  drawing  against  the  ground.* 

*  In  descending  a  steep  hill,  the  wheels  of  a  carriage  are  often  locked  (as 
it  is  called),  that  is,  fastened  in  such  a  manner  as  to  prevent  their  turning; 
and  thus  the  rolling  is  converted  into  the  sliding  friction,  and  the  vehicle 
descends  more  safely. 

Castors  are  put  on  the  legs  of  tables  and  other  articles  of  furniture  to 
facilitate  the  moving  of  them  ;  and  thus  the  sliding  is  converted  into  tha 
rolling  friction. 


86  NATURAL   PHILOSOPHY. 

310.   PRACTICAL  EXAMPLES  OF  POWER  APPLIED  TO  THE  \A  IIREL  AND  AJL». 
Questions  for  Solution. 

(1.)  With  a  wheel  5  feet  in  diameter  and  a  power  of  6  pounds,  whaJ 
nust  be  the  diameter  of  the  axle  to  support  3  cwt.  1  Ans.  1.2  in. 

(2.)  How  large  must  be  the  diameter  of  the  wheel  to  support  with  10 
lb~  a  weight  of  5  cwt.  on  an  axle  0  inches  in  diameter  1  Ans.  81.5ft. 

(3.)  A  wheel  has  a  diameter  of  4  feet,  an  axle  of  6  inches.  What  power 
must  be  applied  tf  the  wheel  to  balance  2  cwt.  on  the  axle  ?  Ans.  25  Ib. 

(4.)  There  is  a  connexion  of  cogged  wheels  having  6  leaves  on  the  pinion 
and  36  cogs  on  the  wheel.  What  is  the  proportion  of  the  power  to  the 
weight  in  equilibrium  *  Ans.  As  1  to  6. 

(5.)  Suppose  a  lever  of  six  feet  inserted  in  a  capstan  2  feet  in  diameter, 
and  six  men  whose  united  strength  is  represented  by  i  of  a  ton  at  the  capstan, 
how  heavy  an  anchor  can  they  draw  up,  allowing  the  loss  of  £  of  their  powe? 
from  friction  }  Ans.  2  T. 

(6.)  What  must  be  the  proportion  of  the  axle  to  the  wheel,  to  sustain  a 
weight  30  cwt.  with  a  power  of  3  cwt.  1  Ans.  As  1  to  10. 

(7.)  The  weight  is  to  the  power  in  the  proportion  of  six  to  one.  What 
must  bo  the  proportion  of  the  wheel  to  the  axle  1  Ans.  6  to  1. 

(8.)  The  power  is  represented  by  10,  the  axle  by  2.  How  can  you  repre- 
sert  the  wheel  and  axle  1  An*.  10 :  weight::  2 :  wheel. 

(9.)  The  weight  is  expressed  by  15,  the  power  by  3.  What  will  repre- 
sent the  wheel  and  axle  7  Ans.  5  and  1. 

(10.)  The  axle  is  represented  by  16,  the  power  by  4.  Required  the  pro 
portion  of  the  wheel  and  axle.  Ans.  4:  weight::  16:  wheel. 

(11.)  What  is  the  weight  of  an  anchor  requiring  6  men  to  weigh  it,  by 
means  of  a  capstan  2  feet  in  diameter,  with  a  lever  8  feet  long,  2  feet  of  ita 
length  being  inserted  in  the  capstan  ;  supposing  the  power  of  each  man  to 
be  represented  by  2  cwt.,  and  a  loss  of  £  the  power  by  friction?  Ans.  b&cwt. 

(12.)  A  stone  weighing  2  tons  is  to  be  raised  by  a  windlass  with  spoked 
2  feet  in  length,  projecting  from  an  axle  9  inches  in  diameter.  How  many 
men  must  be  employed,  supposing  each  man's  power  equal  to  2  cwt.,  and  the  re- 
bistance  increased  j  by  friction  ?  Ana,  5  men. 

What  is  a  311.  THE  PULLEY.  —  The  Pulley  is  a  small 
Pulley 7  wheel  turning  on  an  axis,  with  a  string  or  rope 
in  a  groove  running  around  it. 

How  many  kinds          There    ftre    twQ    kindg    Qf    pulleys  —  the 

of  pulleys     are  ? 

there  i  fixed  and  the  movable.     The  fixed  pulley 

is  a  pulley  that  has  no  other  motion  than  a  revolution  on 
its  axis,  and  it  is  used  only  for  changing  the  direction  of 
motion. 

Explain  312.  Fig.  46  represents  a  fixed  pulley.  P  is  a 
"#•  small  wheel  turning  on  its  axia,  with  a  string  running 

round  it  in  a  groove.  W  is  a  weight  to  be  raised,  F  is  the  force 
t>r  power  applied.  It  is  evident  that,  by  pulling  the  string  at 
F,  the  weight  must  rise  just  as  much  as  the  string  is  drawn 


THE   MECHANICAL   FOWU128.  S7 

down.     As,  therefore,  the  velocity  of  the  weight  and  the   m%.  46 
power  is  precisely  the  same,  it   is   manifest   that  they 
balance  each  other,  and  that  no  mechanical  advantage 
is  gained.*     But  this  pulley  is  very  useful  for  changing 
the  direction  of  motion.     If,  for  instance,  we  wish  to 
raise  a  weight  to  the  top  of  a  high  building,  it  can  be  done 
with  tbe  assistance  of  a  fixed  pulley,  by  a  man  standing 
below.     A  curtain,  or  a  sail,  also,  can  be  raised  by  means  of  a 
fixed  pulley,  without  ascending  with  it,  by  drawing  down  a  string 
running  over  the  pulley. 

On  what  prin-  313.  The  fixed  pulley  operates  on  the  same 
tiple  does  the  principle  as  a  lever  of  the  first  kind  with  equal 
fixed  pulley  act?  armgj  where  the  fu]crum  being  in  the  centre 

of  gravity,  the  power  and  the  weight  are  equally  distant  from  it. 
and  no  mechanical  advantage  is  gained. 

314.     The   movable   pulley   differs    from 
How   does   the  •* 

movable  pulley    the  fixed  pulley  by  being  attached  to  Fig.  47. 

differ  from  the    tne  weight ;  it   therefore  rises    and 
fixed  ? 

falls  with  the  weight. 

Explain  315.  Fig.  47  represents  a  movable  pull ny, 
Fig.  47.  with  the  weight  W  attached  to  it  by  a  hook 
below.  One  end  of  the  rope  is  fastened  at  F ;  and,  as 
the  power  P  draws  the  weight  upwards,  the  pulley 
rises  with  the  weight.  Now,  in  order  to  raise  the 
weight  one  inch,  it  is  evident  that  both  sides  of  the  string 

*  Although  the  fixed  pulley  gives  no  direct  mechanical  advantage,  a 
man  may  advantageously  use  his  own  strength  by  the  use  of  it.  Thus,  if 
he  seat  himself  on  a  chair  suspended  from  one  end  of  a  rope  passing  over  a 
fixed  pulley,  he  may  draw  himself  up  by  the  other  end  of  the  rope  by  exert- 
ing a  force  equal  only  to  one-half  of  his  own  weight.  One  half  of  his  weight 
is  supported  by  the  chair  and  the  other  half  by  his  hands,  and  the  effect  is 
tne  same  as  if  he  drew  only  one  half  of  himself  at  a  time;  for,  the  rope  being 
doubled  across  the  pulley,  two  feet  of  the  rope  must  pass  Ihrough  his  hands 
before  he  can  raise  himself  one  foot.  In  this  manner  laborers  and  others 
frequently  descend  into  wells,  and  from  the  upper  floors  of  stores,  by  meaue 
of  a  rope  passing  over  a  Cxei'  wheel  ( f  pulley. 


88  NATURAL    PHILOSOPHY. 

must  be  shortened ;  in  order  to  do  which,  the  power  P  rnunt 
pass  over  two  inches.  As  the  velocity  of  the  power  is  double 
that  of  the  weight,  it  follows  that  a  power  of  one  pound  will  bal- 
ance a  weight  on  the  movable  pulley  of  two  pounds.1* 

What  is  the  ad-      316.  The  power  gained  by  the  use  of  pul- 

vantage  gained    ,          .  t    .      ,  ,  ,  .   ,    .         . 

in  the  use  of  the    ^eJs  1S  ascertained  by  multiplying  the  num- 

movable pulley ?     ber  of  movable  pulleys  by  2.f 

317.  A  weight  of  72  pounds  may  be  balanced  by  a  power  of  9 
pounds  with  four  pulleys,  by  a  power  of  18  pounds  with  two  pul- 
leys, or  by  a  power  of  3G  pounds  with  one  pulley.  But  in  each 
case  the  space  passed  over  by  the  power  must  be  double  the  space 
passed  over  by  the  weight,  multiplied  by  the  number  of  movable 
pulleys.  That  is,  to  raise  the  weight  one  foot,  with  one  pulley,  the 
power  must  pa^s  over  two  feet,  with  two  pulleys  four  feet,  with 
four  pulleys  eight  feet. 

Explain        318.     Fig.   48  represents   a  sy.stem   of  fixed   and 

8  '  movable  pulleys.  In  the  block  F  there 
are  four  fixed  pulleys,  and  in  the  block  M  there 
are  four  movable  pulleys,  all  turning  on  their  com- 
mon axis,  and  rising  and  falling  with  the  weight 
W.  The  movable  pulleys  are  connected  with  the 
fixed  ones  by  a  string  attached  to  the  hook  H, 
passing  over  the  alternate  grooves  of  the  pulleys 
in  each  block,  forming  eight  cords,  and  terminating 
at  the  power  P.  Now,  to  raise  the  weight  one  foot, 
it  is  evident  that  each  of  the  eight  cords  must  be 

Thus,  it  is  seen  that  pulleys  act  on  the  same  principle  with  the  lever 
and  the  wheel  and  axle,  the  deficiency  of  the  strength  of  the  power  being 
compensated  by  superior  velocity.  Now,  as  we  cannot  increase  our  natural 
strength,  but  can  increase  the  velocity  of  motion,  it  is  evident  that  we  are 
enabled,  by  pulleys,  and  other  mechanical  powers,  to  reduce  the  resistance 
or  weight  of  any  body  to  the  level  of  our  strength. 

f  This  rule  applies  only  to  the  movable  pulleys  in  the  same  block,  or 
when  the  parts  of  the  rope  which  sustains  the  weight  are  parallel  to  each 
other.  The  mechanical  advantage,  however,  which  the  pulley  seems  to  possess 
in  theory,  is  considerably  diminished  in  practice  by  the  stiffness  of  the  ropes 
aud  the  fricMon  of  the  wheels  and  blocks.  When  the  parts  of  the  cord, 
also,  are  not  parallel,  the  pulley  becomes  less  efficacious  ;  and  when  the 
parts  of  the  cord  which  supports  the  weight  very  widely  depart  from  par- 
allelism, the  pulley  becomes  wholly  useless.  There  are  certain  arrange- 
ments of  the  Jord  aud  the  pulley  by  which  the  effective  power  of  tb» 


THE   MECHANICAL   POWEKS.  89 

shortened  one  foot,  and,  consequently,  that  the  power  I*  must 
descend  eight  times  that  distance.  The  power,  therefore,  must 
pass  over  eight  times  the  distance  that  the  weight  moves. 

319.  The  movable  pulley,  as  well  as  the  fixed,  acts  on  the  same 
principle  with  the  lever,  the  deficiency  of  the  strength  of  the 
power  with  the  movable  pulley  being  compensated  by  its  superior 
velocity. 

On  what  princi-  32°-  The  fixed  PulleJ  acts  on  the  principle  of 
pie  is  the  mov-  a  lever  with  equal  arms.  [See  No.  313.]  The 
structed?  ^  '  movakle  Pu^ey>  on  *ne  contrary,  by  giving  a 
superior  velocity  to  the  power,  operates  like  a 
lever  with  unequal  arms. 

321.  Practical  use  of  Pulleys.  —  Pulleys  are  used  to  raise  goods 
into  warehouses,  and  in  ships,  &c.,  to  draw  up  the  sails.    Both  kind? 
of  pulleys  are  in  these  causes  advantageously  applied  :  for  the  sails 
are  raised  up  to  the  xuasts  by  the  sailors  on  deck  by  means  of  the 
fixed  pulleys,  while  the  labor  is  facilitated  by  the  mechanical  power 
of  the  movable  ones. 

322,  Both  fixed  and  movable  pulleys  are  constructed  in  a  great 
variety  of  forms,  but  the   principle  on  which  all  kinds  are  con- 
structed is  the  same.     What  is  generally  called  a  tackle  and  fall ', 
or  a  block  and  tackle,  is  nothing  more  than  a  pulley.     Pulleys  have 
likewise  lately  been  attached  to  the  harness  of  a  horse,  to  enable 
the  driver  to  govern  the  animal  with  less  exertion  of  strength 

323.  It  may  be  observed,  in  relation  to  the  Me- 
What  law  ap-  .  J        , 

plies  to  all  the    chanical  Powers  in  general,  that  power  is  always 

Mechanical*  gained  at  the  expense  of  time  and  velocity  ;  thai 
is,  the  same  power  which  will  raise  one  pound  in 
one  minute  will  raise  two  pounds  in  two  minutes,  six  pounds  in 
six  minutes,  sixty  pomids  in  sixty  minutes,  <J-c. :  and  that  the 
same  quantity  of  force  used  to  raise  two  pounds  one  foot  will 
raise  one,  pound  two  feet,  fyc.  And,  further,  it  may  be  stated 
that  the  product  of  the  weight  multiplied  by  the  velocity  of  the 
weight  will  always  be  equal  to  the  product  of  the  power  multi 
plied  by  tke  velocity  of  the  power. 

Siihey  mny  be  augmented  in  a  three-fold  instead  of  a  two-fold  proportion. 
u.t  when  such  an  advantage  is  secured,  it  must  be  by  contriving  to  make 
the  power  pass  over  three  times  the  space  of  the  weight. 
*  See  Appendix. 


'JO  NATURAL    PHILOSOPHY. 

In  what  proper-        Hence  we  have  the  following  rule .    The 

tton  is  the  power  7 

to    the    weight    Power  ls  M  **&  same  proportion  to  trie 

when   the  mov-    weight  as  the  velocity  of  the  weight  is  tn 

able    pulley     is     j7          ,     .,        -     7 

used  i  Me  velocity  of  the  power.* 

324.   PRACTICAL  EXAMPLES  OF  APPLICATION  OF  THE  PULLET. 
Questions  for  Solution. 

(1.)  Suppose  a  power  of  9  Ibs.  applied  to  a  set  of  3  movable  pulleys.  Ai 
lowing  }  loss  for  friction,  what  weight  can  be  sustained  by  them  1  A.  36  Ib. 

(2.)  Six  movable  pulleys  are  attached  to  a  weight  of  1800  Ibs.;  what 
power  will  support  them,  allowing  a  loss  of  two-thirds  of  the  power  from 
friction  1  Ans.  4,50  Ib. 

(3.)  Six  men,  with  a  block  and  tackle  containing  nine  movable  pulltys, 
ars  required  to  raise  a  sail.  Suppose  each  man's  strength  to  be  represented 
by  two  cwt.  and  two-thirds  of  the  power  lost  by  friction,  what  is  the 
weight  of  the  sail,  with  its  appendages  1  Ans.  72  cwt. 

(4.)  If  a  stone  weighing  3  tons  is  to  be  raised  by  horse  power  to  the  wall 
of  a  building  in  process  of  erection,  by  means  of  a  derrick  from  which  are 
suspended  3  movable  pulleys,  how  many  horses  must  be  employed,  sup- 
posing each  horse  capable  of  drawing  as  much  as  eight  men,  each  of  whom 
can  lift  2  cwt.,  making  an  allowance  of  two-thirds  for  friction  1  Ans.  1£.  ' 

(5.)  A  block  contains  5  movable  pulleys,  connected  with  a  beam  contain- 
ing 5  fixed  pulleys.  A  weight  of  half  a  ton  is  to  be  raised.  Allowing  a  loss 
of  two- thirds  for  friction,  what  power  must  be  applied  to  raise  it  1  A.  3  cwt. 

(7.)  The  power  is  3,  the  weight  is  27;  how  many  pulleys  murft  be  usea, 
if  friction  requires  an  allowance  of  two-thirds  1  Ana.  27, 

(8.)  Friction  one-third  of  the  power,  power  6,  weight  72,  —  how  many  pul- 
leys 1  Ans.  IS. 

(9.)  Weight  84,  friction  nothing,  pulleys,  3  fixed,  3  movable  ;  required 
the  power.  Ans.  14 

(10.)  Power  12,  friction  8,  four  pulleys,  two  of  them  fixed  ;  required  the 
weight.  Ans.  16. 

(11.)  Six  movable  and  six  fixed  pulleys.  The  weight  is  raised  3  feet. 
How  far  has  the  power  moved  '!  Ans.  36/t 

,12.)  The  power  has  moved  12  feet  ;  how  far  has  the  weight  moved  un- 
der two  pulleys,  one  fixed,  the  other  movable  1  Ans.  &ft. 

(13.)  The  weight,  suspended  from  a  fixed  pulley,  has  moved  6  feet.  How 
far  has  the  power  moved  1  Ans.  6ft. 

(14.)  The  power  has  moved  2-0  feet  under  a  fixed  pulley  ;  how  far  haa 
the  weight  moved  *  Ans.  20ft. 

What  is  the  In-  325.  THE  INCLINED  PLANE. —  The  In- 
dined  Plane?  cjine(i  Plane  consists  of  a  hard  plain  surface, 
inclined  to  the  horizon. 

326.  The  principle  on  which  the  inclined  plane  acts  as  a  me- 
"rmnical  power  is  simply  the  fact  that  it  supports  part  of  the  weight. 
[f  a  body  be  placed  on  a  horizontal  plane,  its  whole  weight  will  be 

*  The  stiffness  of  the  cords  and  the  friction  of  the  blocks  frequentlj 
require  large  deduction  to  be  made  from  the  effective  power  of  pulleys 
TMu  loss  thus  occasioned  will  sometimes  amount  to  two-thirds  of  the  p'.vrcr 


THE   MECHANICAL   POWERS.  91 

supported  ,  but,  if  the  plane  be  elevated  at  one  end,  by  degrees,  it 
will  support  less  of  the  weight  in  proportion  to  the  elevation,  until 
the  plane  becomes  at  right  angles  to  the  horizon,  when  it  will  sup- 
port no  part  of  the  weight,  and  the  body  will  fall  perpendicularly. 

^27.  A  body,  in  ascending  or  descending  an  inclined  plane,  wiL 
ha  ye  a  greater  space  to  traverse  than  if  it  should  rise  or  fall  per- 
pendicularly. The  time,  therefore,  of  its  ascent  or  descent  will  be 
longer,  and  thus  it  will  oppose  less  resistance,  and  thus,  also,  a  less 
force  will  be  required  to  cause  its  ascent.  Hence,  we  see  that  the 
fundamental  principle  of  Mechanics,  "  What  is  gained  in  power  is 
lost  in  time,"  applies  to  the  Inclined  Plane  as  well  as  to  the  Me- 
chanical Powers  that  have  already  been  described. 

What  is  the  ad-      328.  The  advantage  gained  by  the  use  of 

vantage  gained       ,  ,.      ,      , 

by  the  use  of  the    tne  inclined  plane  is  in  proportion  as  the 

inclined  plane  ?  length  of  the  plane  exceeds  its  perpen- 
dicular height. 

Fig.  49  represents  an  inclined  plane.  C  A  its  height,  C  B 
its  length,  and  W  a  weight  which  is  to  be  K  49 

moved  on  it.  If  the  length  C  B  be  four 
times  the  height  C  A  then  a  power  of  one 
pound  at  G  will  balance  a  weight  of  four 
pounds  on  the  inclined  plane  C  B. 

329.  The  greater  the  inclination  of  the  plane,  the  greater  must 
be  its  perpendicular  height,  compared  with  its  length  ;    and,  of 
course,  the  greater  must  be  the  power  to  elevate  a  weight  along  its 
surface. 

330.  Instances  of  the  application  of  the  inclined  plane  are  very 
common.     Sloping  planks  or  pieces  of  timber  leading  into  a  cellar, 
and  on  which  casks  are  rolled  up  and  down ;  a  plank  or  board  with 
one  end  elevated  on  a  step,  for  the  convenience  of  trundling  wheel- 
barrows, or  rolling  barrels  into  a  store,  &c.,  are  inclined  planes. 

331.  Chisels  and  other  cutting  instruments,  which  are  cham- 
fered, or  sloped  only  on  one  side,  are  constructed  on  the  principle 
of  the  inclined  plane.* 

332.  Roads  which  are  not  level  may  be  considered  as  inclined 
planes,  and  the  inclination  of  the  road  is  estimated  by  the  height 
corresponding  to  some  proposed  length.     To  raise  a  load  up  an 
inclined   plane  requires  a  power  sufficient   to  carry  it  along  the 
whole  distance  of  the  length  of  the  base,  and  then  to  lift  it  up  to 

*  Chisels  for  cutting  wood  should  have  their  edges  at  an  angle  of  about 
30°  ;  for  cutting  ;ron  from  50  J  to  60°,  and  for  cutting  brass  at  about  80°  or 
90°.  Tools  urged  by  pressure  may  be  sharper  than  those  which,  like  the 
wedge,  are  diiven  by  percussion 

4* 


93  NATUKAL   PHILOSOPHY. 

the  elevation  ;  but  in  the  inclined  plane  a  feebler  force  will  accom- 
plish the  desired  object,  because  the  resistance  is  spread  equally 
over,  the  whole  distance.* 

What  is  the      333.  THE  WEDGE.—  The  Wedge   consists 

ff;|7       7  * 

of  two  inclined  planes  united  at  their  bases. 
What  is  the  ad-      334.  The  advantage  gained  by  the  wedge 
ls  in  Proportion,  as  its  length  exceeds   the 
thickness  between  the  converging  sides. 

In   what  pro- 

portion is  the        It  follows  that  the  power  of  the  wedge  ia  in  pio- 

poiver  of  the    portion  to  its  sharpness. 

wedge  ? 

835.  Fig.  50  represents  a  wedge.  The  line  a  b 
represents  the  base  of  each  of  the  inclined  planes 
of  which  it  is  composed,  and  at  which  they  are 

united. 

b 

336.  The  wedge  is  a  very  important  mechanical  power,  used  to 
split  rocks,  timber,  &c.,  which  could  not  be  effected  by  any  othei 
power.f 

337.  Axes,  hatchets,  knives,  and  all  other  cutting  instruments, 
chamfered,  or  sloped  on  both  sides,  are  constructed  on  the  principle 
of  the  wedge;  also  pins,  needles,  nails,  and  all  piercing  instru- 
ments. 

On  what  does      338.  The  effective  power  of  the  wedge  depends 
the    on  friction  5   for»  tf  there  were  no  Diction,  the 


wedge  depend  ?    wedge  would  fly  back  after  every  stroke. 

*  Mention  has  already  been  made  of  the  sagacity  of  animals  in  a  former 
page  [see  No.  54],  and  a  sort  of  intuitive  knowledge  which  they  appear 
to  possess  of  philosophical  principles.  In  ascending  a  steep  hill,  a  common 
dray-horse  will  drag  his  load  from  side  to  side,  as  if  he  were  conscious  that 
be  thus  made  the  plane  longer  in  proportion  to  its  height,  and  thereby 
made  his  load  the  lighter. 

t  The  wedge  is  an  instrument  of  exceedingly  effective  power,  and  la 
frequently  used  in  presses  for  extracting  the  juice  of  seeds,  fruits,  <fco.  It 
Is  used  especially  in  the  oil  mill,  by  which  the  oil  is  extracted  from  seeds. 
The  seeds  are  placed  in  hair  bags,  between  planes  of  hard  wood,  which  are 
pressed  together  by  wedges.  The  pressure  thus  exerted  is  so  intense  that 
the  seeds,  after  the  extraction  of  the  -oil,  are  converted  into  masses  as  hard 
and  compact  as  the  most  dense  woods. 

Wedges  are  used  also  in  the  launching  of  vessels,  and  also  for  restoring 
buildings  to  the  perpendicular  which  have  been  inclined  by  the  sinking  of 
the  foundation. 


THE   MECHANICAL   POWERS.  93 

339.  The  wedge  derives  much  of  its  efficiency  from  the  force  of 
percussion,  which  in  its  nature  is  so  different  from  continued  force, 
such  as  the  pressure  of  weights,  the  force  of  springs,  &c.,  that  it 
would  be  difficult  to  submit  it  to  numerical  calculation  ;  and,  there- 
fore, we  cannot  properly  represent  the  proportion  which  a  blow 
bears  to  the  weight. 

What: 'Ufa        340.     THE  SCREW.  — The  Screw  is  an  in- 
Screw?  clined  plane   wound  around  a   cylinder,   thus 

producing  a  circular  inclined  plane,  forming  what  is  called 
the  threads  of  the  screw. 

341.  Cut  a  piece  of  paper  in  the  shape  of  an  inclined  plane,  as 
represented  by  Fig.  49,  and,  beginning  with  the  end  represented 
by  the  height  C  A,  in  that  Figure,  wind  it  around  a  pencil,  or  a 
round  ruler.  The  edge  of  the  paper  will  be  a  circular  inclined  plane, 
and  will  represent  the  threads  of  the  screw.  The  distance  between 
any  two  threads  on  the  same  side  of  the  rule  will  represent  the  per- 
pendicular height  of  the  inclined  plane  that  extends  once  around  the 
cylinder,  and  the  advantage  gained  in  the  use  of  the  screw  (when 
used  without  a  lever)  will  be  the  same  as  in  the  inclined  plane ; 
namely,  as  the  length  of  the  plane  exceeds  the  perpendicular  height. 
But  the  screw  is  seldom  used  alone.  A  lever  is  generally  attached 
to  the  screw,  and  it  is  with  this  attachment  the  screw  will  now  be 
considered. 

342.     The   Screw  is  generally  accompanied 
What  appendage  J 

generally  attends     by  an  appendage  called  the  nut,  which  consists 

the  Screw  ?  Of  a  concave  cylinder  or  block,  with  a  hollow 

spiral  cavity  cut  so  as  to  correspond  exactly  with  the  threads  of 
the  screw.  When  thus  fitted  together,  the  screw  and  the  nut 
form  two  inclined  planes,  the  one  resting  on  the  other. 

343.     Sometimes   the   screw  is   movable  and 
Is  the  screw,  or 
the  nut    mov-    the  nut  is  stationary,  and  sometimes  the  screw 

able  ?  is  stationary  and  the  nut  is  movable. 

344.  At  every  revolution  the  screw  or  the  nut  advances  or 
retreats  through  a  *Dace  equal  to  the  distance  between  the  threads 
of  the  screw.  • 

In  what  manner  345.  The  power  applied  to  a  screw  gener- 
%epliede  to°Wthe  allv  Describes  a  circle  around  the  screw, 
screw  move?  perpendicular  to  the  direction  in  which 
the  screw  or  nut  moves. 


NATURAL    PHILOSOPHY. 


\Tn<A  is  the  advan-  346«  The  advantage  gained  by  the 
*  gained  by  the  screw  is  in  proportion  as  the  circumfer- 
ence described  by  the  power  exceeds  the 
distance  between  the  threads  of  the  screw. 


What  is  meant  by       347.     The  cylinder  with  its  threads  is  called 

the  Convex  and        the  Convex  Screw,  and  the  nut  is  called  the 
Concave  Screw?       /,  a  m,      , 

Concave  bcrew.     The  lever  is  sometimes  at- 


Fig.  51. 


tached  to  the  screw,  and  sometimes  to  the  nut. 
Explain        348.    Fig.  51  represents  a  fixed  screw 

*&'      '     S,  with  a  movable  nut  N,  to  which  is 
attached  the  lever  L.     By  turning  the  lever  in  one 
direction  the  nut  descends,  and  by  turning  it  in  the 
opposite  direction  the  nut  ascends,  at  every  revo- 
lution of  the  lever,  through  a  space  equal  to  the  dis- 
tance between  the  threads  of  the  screw  ;  to  accomplish  which,  the 
hand  or  power  applied  to  the  end  of  the  lever  L  will  describe  a 
circle  around  the  sorew  S,  of  which  the  radius  is  L  S.     The 
power  thus  passes  over  a  space  represented  by  the  circumfer- 
ence of  this  circle,  and  the  advantage  gained  is  in  the  same  pro- 
portion as  the  space  exceeds  the  distance  between  each  threa 
of  the  screw 
Explain        349.     Fig.  52  represents  a  movable 

I^P"  screw,  with  a  nut  fixed  in  a  frame,  and 

consequently  immovable.  As  the  lever  L  is 
turned,  the  screw  ascends  or  descends  at  every 
revolution  of  the  lever  through  a  space  equal  to 
the  distance  between  the  threads  of  the  screw,  and 
the  advantage  gained  is  in  the  same  proportion  as  in  the  case  of 
the  movable  nut  in  Fig.  51. 

350.  It  will  thus  be  seen  that,  although  the  screw  is  usually  con- 
sidered distinctly  as  a  mechanical  power,  it  is  in  fact  a  compound 
power,  consisting  of  two  circular  inclined  planes,  moved  by  a  lever. 

351.  The  power  of  the  screw  being  estimated  by  the  distance 
between   the   threads,  it  follows    that  the  closer  the    threads   are 
toother,   the  greater  will  be  the  power,  but  the  slower  will  be  the 
motion  produced :  lor.  every  revolution  of  the  lever  advances  the 


Fig.  52. 


TliE   MLCHAJV1CAL 


screw  01  the  nut  only  through  a  space  as  great  as  the  distance  of  the 
threads  from  each  other. 

352  The  screw  is  applied  to  presses  and  engines  of  all  kinds 
where  great  power  is  to  be  applied,  without  percussion,  through 
small  distances  It  is  used  in  bookbinders'  presses,  in  oider  and 


Pig.  53. 


wme  presses,  in  raising  buildings.  It  is  also  used  for 
coining,  and  for  punching  square  or  circular  holes 
through  thick  plates  of  metal.  When  used  for  this 
purpose,  the  lever  passes  through  the  head  of  the 
screw  and  terminates  at  both  ends  with  heavy 
balls  or  weights,  the  momentum  of  which  adds  to 
the  force  of  the  screw,  and  invests  it  with  immense 
power. 

353.  HUNTER'S  SCREW.  —  The  ingenious  contrivance  known  by 
the  name  of  Hunter's  Screw  consists  of  two  screws  of  different 
threads  playing  one  within  the  other  ;  and  such  will  be  the  effect,  that 
while  one  is  advancing  forward  the  other  will  retreat,  and  the  resist- 
ance will  be  urged  forward  through  a  distance  equal  only  to  the 
difference  between  the  threads  of  the  two  screws.  An  indefinite 
increase  in  the  power  is  thus  obtained,  without  diminishing  the 
thread  of  the  screw.* 

*  From  what  has  been  stated  with  regard  to  the  Mechanical  Powers,  it 
appears  that  by  their  aid  a  man  is  enabled  to  perform  works  to  which  hif 
unassisted  natural  strength  is  wholly  inadequate.  But  the  power  of  all 
machines  is  limited  by  the  strength  of  the  materials  of  which  they  are  com- 
posed. Iron,  which  is  the  strongest  of  all  substances,  will  not  resist  a  strain 
beyond  a  certain  limit.  Its  cohesive  attraction  may  be  destroyed,  acd  it 
can  withstand  no  resistance  which  is  stronger  than  its  cohesive  attraction. 
Besides  the  strength  of  the  materials,  it  is  necessary,  also,  to  consider  the 
time  which  is  expended  in  the  application  of  mechanical  assistance.  Archim- 
edes is  said  to  have  boasted  to  Hiero,  King  of  Syracuse,  that,  if  he  would 
give  him  a  place  to  stand  upon,  he  would  move  the  whole  world.  In  order 
to  do  this,  Archimedes  must  himself  have  moved  over  as  much  more  space 
than  ho  moved  the  world  as  the  weigh:  of  the  world  exceeded  his  own  weight; 
and  it  has  been  computed  that  he  must  have  moved  with  the  velocity  of  a 
cannon-ball  for  a  million  of  years,  in  <  r  ler  to  mov.  the  earUi  the  wtiiiy 
seven  millionth  part  of  an  inch. 


93  NATURAL   PHILOSOPHY. 

?54.  PIUCTICAL  EXAMPLES   OF  THE  APPLICATION   OF  THE  INCLINED  PI.ANII 
AND  THE  SCRJSW. 

Questions  for  Solution. 

(1  )  With  an  inclined  plane  the  power  moves  13  feet,  the  power  is  to  the 
V;eight  as  6  to  24.  How  far  does  the  weight  more  1  Ana.  4  ft. 

(2.)  The  length  of  an  inclined  plane  is  5  feet,  the  proportion  of  the 
^ower  to  the  weight  is  as  2  to  10.  What  is  the  height  of  the  plane  1  A.  I  ft. 

(3.)  An  inclined  plane  is  4  feet  high,  a  power  of  6  Ibs.  draws  up  30 
ibs.  What  is  the  length  of  the  plane  "  An*.  20,/fc 

(4.)  The  length  of  a  plane  is  12  feet,  the  height  is  3  feet.  What  is  the 
proportion  of  the  power  to  the  weight  to  DC  raised  1  An*.  As  1  to  4. 

(5  *  The  distance  between  the  threads  tf  a  screw  is  1  inch,  the  length  of 
the  lever  is  2  feet.  What  is  the  proportion  An*.  1  to  150.79  -f- 

(6.)  Which  will  exert  the  greater  force,  a  lever  3  feet  long  with  the 
fulcrum  6  inches  from  one  end,  or  a  screw  with  a  distance  of  1  inch  between 
the  threads  and  a  lever  one  foot  long  ]  Ana.  The  screw. 

(7.)  A  screw  with  the  threads  2  inches  apart,  and"  a  lever  6  feet  long, 
draws  a  ship  of  200  tons  up  an  inclined  plane  whose  length  is  to  the  height  in 
the  proportion  of  1  to  16.  What  power  must  be  applied  to  the  lever  of  the 
rcrew  !  Ans.  11.05  Ib.  + 

(8.)  If  a  man  can  lift  a  weight  of  150  Ibs.,  how  much  can  he  draw  up  an 
inclined  plane  whose  length  is  to  its  height  as  24  to  3?  .  Ans.  1200  Ib. 

(9.)  A  Hunter's  screw  has  a  lever  four  feet  long.  The  distance  between 
the  threads  of  the  larger  screw  is  1  inch,  between  those  of  the  smaller  i  of  an 
inch.  How  much  weight  can  a  man  whose  power  is  represented  by  175  Ibs. 
move  with  such  a  screw  \  Ans.  211115.52  Ib. 

(10.)  A  screw  with  a  lever  of  2  feet  in  length,  and  a  distance  of  j  of  ail 
inch  between  its  threads,  acts  on  the  teeth  or  cogs  of  a  wheel  whose  diameter 
is  to  that  of  the  axle  as  4  to  1.  Fastened  to  the  axle  is  a  rope,  one  end  of 
which  is  attached  to  a  weight  at  the  bottom  of  an  inclined  plane,  the  length 
of  which  is  to  the  height  as  12  to  3.  Suppose  this  weight  to  require  the 
strength  of  a  man  who  can  lift  200  Ibs.  to  be  applied  to  the  lever  of  the 
screw  to  move  it.  What  is  the  weight  1  Ans.  965099.5200  Ib. 

What  is  the  355.  THE  KNEE  JOINT,  OR  TOGGLE 
Toggle  Joint't  JOINT<  _  The  Toggle  Joint,  or  Knee  Joint, 
consists  of  two  bars  united  by  a  hinge  or  ball  and  socket, 
which,  being  urged  by  a  power  perpendicular  to  the  resistance^ 
acts  with  rapidly-increasing  force,  until  the  bars  form  a 
straight  line 

The  toggle  (or  knee)  joint  affords  a  very  useful  mode  of  convert- 
ing velocity  into  power,  the  motion  produced  being  very  nearly  at 
right  angles  with  the  direction  of  the  force.  It  is  a  combination 
of  levers,  and  the  same  law  applies  to  it  as  to  all  machinery, 
namely,  that  the  power  is  to  the  resistance  inversely  as  the  space 
of  the  power  is  to  the  space  of  the  resistance. 


THE   MECHANICAL    POWERS. 


Explain        356.  Fig  55  represents  a  toggle  joint. 

nected  by  a  joint  at  C.     A  moving  force  applied 
it  0,  in  the  direction  C  D,  acts  with  great  and    1)- 
constantly  increasing  power  to  separate  the  parts 
A  and  B. 

357.  The  operation  of  the  toggle  '*•  56. 
joint  is  seen  in  the  iron  joints  which 

are  used  to  uphold  the  tops  of  chaises. 
It  is  also  used  in  various  kinds  of 
printing-presses  to  obtain  the  great- 
est power  at  the  moment  of  impres- 
sion.* 

358.  MEDIA.  —  The  motion  of  all 
bodies  is  affected  by  the  substance  or 
element  in  which  they  move,  and  by 
which  they  are  on  all  sides  surround- 
ed.    Thus  the  bird  flies  in  the  air,  the 
fish  swims  in  the  water.     Air  there- 
fore is  the  medium  in  which  the  for- 
mer moves,  while  water  is  the  medium 
in  which  the  motion  of  the  latter  is 
aiade. 

What  is  a       359.  A  Medium  is  the  substance,  solid  or  fluid, 
which  surrounds  a  body,  and  which  the  body  must 
displace  as  it  moves. 

360.  When  the  fish  swim?  or  the  bird  flies,  each  must  force  ita 
way  through  the  air  or  the  water  ;  and  the  element  thus  displaced 
must  rush  into  the  spot  vacated  by  the  body  in  its  progress.  It  has 
already  been  stated  that  the  body  of  the  fish  or  of  the  bird  is  pro- 
pelled in  its  motion  in  the  one  case  by  the  reaction  of  the  air  on  tho 
wings  of  the  bird,  and  in  the  other  of  the  water  on  the  fins  of  a  fish 
The  fish  moves  in  the  denser  medium  and  needs  therefore  to  present 
a  less  surface  for  the  reaction  of  the  water  ;  while  the  bird,  living  in 
a  comparatively  rare  medium,  presents  in  his  wings  a  much  larger 
extent  of  surface  to  receive  the  reaction  of  the  air.  In  making 
the  fins  of  a  fish,  therefore,  so  much  smaller,  in  proportion  to  its 
size,  than  the  wings  of  a  bird,  nature  herself  has  taught  us  that, 


In  wiiat  proportion 
is  ths  resistance  of  a 
medium  ? 


361.  The  resistance  of  a  medium  is 
in  exact  proportion  to  its  density. 


*  A  similar  effect,  but  with  a  reversed  action,  is  produced  when  a  long  rope, 
tightly  strained  between  two  points,  is  forcibly  pulled  iu  the  middle 


98  NATUKAL   PHILOSOPHY 

302.  A  body  falling  through  water  will  move  more  slo\\  ly  than 
one  falling  in  the  air.  because  it  meets  with  more  resistance  from 
the  inertia  of  the  water,  on  account  of  the  greater  density  of  the 
water. 

What  is  a  363.  A  VACUUM.  —  A  Vacuum  is  unoccu- 

Vacuumf         pje(j  space  ;    that  is,  a  space  which  contain* 
absolutely  nothing. 

364.  From  this  definition  of  a  vacuum,  it  appears  that  it  does 
not  mean  a  space  which  to  our  eyes  appears  empty.     What  we  call 
an  empty  bottle  is,  in  fact,  full  of  air,  or  some  other  invisible  fluid. 
If  we  sink  an  empty  bottle  in  water  or  any  other  liquid,  neither  the 
water  nor  any  other  liquid  can  enter  until  some  portion  of  the  air  is 
expelled.     A  small  portion  of  water  enters  the  bottle  immersed, 
and  the  air  issues  in  bubbles  from  the  mouth  of  the  bottle.     Other 
portions  of  water  then  enter  the  bottle,  expelling  the  air  in  similar 
manner,  until  the  water  entirely  fills  the  bottle,  and  then  the  air 
bubbles  cease  to  rise. 

365.  From  this  statement  of  the  meaning  of  the  term  "  a  vacuum" 
it  will  be  seen  that  if  a  machine  be  worked  in  a  vacuum  (or,  as  it 
is  more  commonly  expressed  in  Latin,  "  in  vacua  ")  its  motion  will 
be  rendered  easier,  because  the  parts  receive  no  resistance  from  a 
surrounding  medium. 


What  is  Fn'c-        *^*  FRICTION.-  —  Friction  is  the  resistance 
tion,  and  how    which  bodies  meet  with  in  rubbing   against 

' 


there?    De-          There  are  two  kinds  of  friction,  namely, 
scribe  each.         ^    rolling    and    ^    gliding    friction>       The 

rolling  friction  is  caused  by  the  rolling  of  a  circular  body. 

36T.  The  sliding  friction  is  produced  by  the  sliding  or 
dragging  of  one  surface  over  another. 

368.  Friction  is  caused  by  the  unevenness  of  the  surfaces  which 
come  into  contact.*  It  is  diminished  in  proportion  as  the  surfaces 
are  smoothed  and  well  polished.  The  sliding  friction  is  overcome 
with  more  difficulty  than  the  rolling. 


*  All  bodies,  how  well  soev«r  they  may  bo  polished,  have  inequalities  in 
their  surfaces,  which  may  be  perceived  by  a  microscope.  When,  therefore, 
the  surfaces  of  two  bodies  come  into  contact,  the  prominent  parts  of  the 
one  will  often  fall  into  the  hollow  parts  of  the  other,  aud  cause  more  w 
l.'ss  ret  Stance  to  motion. 


THE   MECHANICAL  POWERS.  99 

What  portion          369.  Friction  destroys,  but  never  can  gen- 

of  the  power  of      r  ^     motion.     It  is  frequently  computed 

a  machine  is  lost  J  r 

by  friction  f         that  friction  destroys  one-third  of  the  power 

of  a  machine.  In  calculating  the  power  of  a  machine, 
therefore,  an  allowance  of  one- third  must  be  made  for  loss 
by  friction.* 

370.  Oil,  grease,  black-lead  or  powdered  soap- 
What  is  used 
to  lessen  fric-     stone,  is  used  to  lessen  friction,  because  they  act 

tion?  and         ag   a   polish   by   filling  up   the   cavities  of  the 
9 '  rubbing  surfaces,  and  thus  make  them  slide  more 

easily  over  each  other. 

How  does  fric-        371.  Friction  increases : 

lion  increase  ?         (1.)  A.S  the  weight  or  pressure  is  increased. 
(2.)  As  the  extent  of  the  surfaces  in  contact  is  increased 
(3.)  As  the  roughness  of  the  surface  is  increased. 

How  may  fric-        372«  Friction  maJ  be  diminished  : 

lion  be  dimin-         (1.)  By  lessening  the  weight  of  the  body  in 

ished  ?  motion. 

(2.)  By  mechanically  reducing  the  roughness  of  the  sliding 
Kurfaces. 

(3.)  By  lessening  the  amount  of  surface  of  homogeneous 
bodies  in  contact  with  each  other. 

(4.)  By  converting  a  sliding  into  a  rolling  motion. 

(5.)  By  applying  some  suitable  unguent.t 

*  "When  finely-polished  iron  is  made  to  rub  on  bell-metal,  the  friction  is 
said  to  be  reduced  to  about  one-eighth.  Mr.  Babbit,  of  Boston,  has  pre- 
pared a  composition  for  the  wheel-boxes  of  locomotive  engines  and  other 
machinery,  which,  it  is  said,  has  still  further  reduced  the  amount  of  fric- 
t'on.  This  composition  is  now  much  in  use.  As  the  friction  between 
rolling  bodies  is  much  less  than  in  those  that  drag,  the  axle  of  large  wheels 
is  sometimes  made  to  move  on  small  wheels  or  rollers.  These  are  called 
friction  wheels,  or  friction  rollers.  They  turn  round  their  own  centre  as 
the  wheel  continues  its  motion. 

t  From  the  experiments  made  by  Coulomb,  it  appears  that  the  friction 
of  heterogeneous  ;  bodies  is  generally  less  than  that  of  homogenous  that 
Is,  that  if  a  body  rub  against  another  composed  of  the  same  kind  of  wood 
•v>r  metal,  the  friction  is  greater  than  that  of  different  kinds  of  metal,  or  of 
wood. 

Ferguson's  experiments  go  to  prove  that  the  friction  of  polished  Pt«e\ 
against  polished  $U»el  \s  greater  than  ttmt  of  rolished  steel  on  cupper  or  on 


100  NATURAL   PHILOSOPHY. 

What  cure  the  373.  Friction,  although  it  retards  the  motion 

uses  offnction  f  of  machines,  and  causes  a  great  loss  of  power, 
performs  important  benefits  in  full  compensation.  Were  there 
no  friction,  all  bodies  on  the  surface  of  the  earth  would  be  clash- 
ing against  each  other.  Kivers  would  dash  with  unbounded 
velocity,  and  we  should  see  little  but  motion  and  collision.  But 
whenever  a  body  acquires  a  great  velocity,  it  soon  loses  it  by 
friction  against  the  surface  of  the  earth. 

374.  The  friction  of  water  against  the  surfaces  it  runs  over  soun 
reduces  the  rapid  torrent  to  a  gentle  stream  ;  the  fury  of  the  tempest 
is  lessened  by  the  friction  of  the  air  on  the  face  of  the  earth  ;  and 
the  violence  of  the  ocean  is  soon  subdued  by  the  attrition  of  its  own 
ivaters.  Our  garments,  also,  owe  their  strength  to  friction  ;  and 
the  strength  of  ropes,  cords,  sails  and  various  other  things,  depends 
on  the  same  cause,  for  they  are  all  made  of  short  fibres  pressed 
together  by  twisting,  and  this  pressure  causes  a  sufficient  degree  of 
friction  to  prevent  the  fibres  sliding  one  upon  another.  Without 
friction  it  would  be  impossible  to  make  a  rope  of  the  fibres  of  henip, 
or  a  sheet  of  the  fibres  of  flax ;  neither  could  the  short  fibres  of 
cotton  have  ever  been  made  into  such  an  infinite  variety  of  forms  as 
they  have  received  from  the  hands  of  ingenious  workmen.  Wool, 
also,  has  -been  converted  into  a  thousand  textures  of  comfort  and 
luxury,  and  all  these  are  constituted  of  fibres  united  by  friction. 

What  is  the  375.      REGULATORS     OF     MOTION.  —  TlIE 

Pendulum  ?      PENDULUM.  —  The  Pendulum  *  consists  of  a 


brass.  la  a  combination  where  gun-metal  rubs  against  steel,  the  same 
weight  may  be  moved  with  a  force  of  fifteen  and  a  half  pounds  that  it 
would  require  twenty -two  pounds  to  move  when  cast-iron  moves  against 
steel. 

*  The  pendulum  was  invented  by  Galileo,  a  great  astronomer  of  Florence, 
in  the  beginning  of  the  seventeenth  century.  Perceiving  that  the  chan 
deliers  suspended  from  the  ceiling  of  a  lofty  church  vibrated  long  and  with 
great  uniformity,  as  they  were  moved  by  the  wind  or  by  any  accidental 
disturbance,  he  was  led  to  inquire  into  the  cause  of  their  motion,  and  this 
inquiry  led  to  the  invention  of  the  pendulum.  '  From  a  like  apparently 
insignificant  circumstance  arose  the  great  discovery  of  the  principle  of 
gravitation.  During  the  prevalence  of  the  plague,  in  the  year  iCtio,  Sir 
Isaac  Newton  retired  into  the  country  to  avoid  the  contagion.  Sitting  in 
his  orchard,  one  day,  he  observed  an  apple  fall  from  a  tree.  His  inquisitive 
inind  was  immediately  led  to  consider  the  cause  'vhich  brought  the  apple 
to  the  ground,  and  the  result  of  his  inquiry  was  the  discovery  of  that  grand 
principle  of  gravitation  which  may  be  considered  as  the  first  arid  most  im- 
portant law  of  material  nature.  Thus,  out  of  what  had  been  before  the 
eyes  of  men,  in  one  shape  or  another,  from  the  creation  of  the  w>rjkl,  di<? 
ihe.3i.  pnilos  jpbers  bring  the  most  important  results. 


KEGULATOBS    OF   MOTION.  101 

weight  or  ball  suspended  by  a  rod,  and  made   to  swing 
backwards  and  forwards. 

What  are  the 

motions  of  a          376.  The  motions  of  a  pendulum  are  called 

pendulum  call-  fa  vibrations   or   oscillations,   and   they  are 

°d,  and  how 

ire  they  caused  by  gravity.* 

caused  ? 

What  is  the          The  part  of  a  circle  through  which  it  movess 
arc  of  a  pend-    .        n    i  -^  '».>•*, 

ulum?  1S  called  lts  arc'  '•„'•  •:•  '   V  .  .  ,  *' 

What  differ-         377.  The  vibrations  of. yenfo  lions  pf  < 
^ke  time^of  the  lengtn  are  verj  nearly  equal,  '^fcetner*  ^^ 
vibrations  of     move  through  a  greater  or  less    part  of  thcii 

pendulums  of  i 

e^ual  length?     arCS't 

378.  In  Fig.  57  A  B  represents  a  pendulum5         «K.  57. 
DFEC  the  arc  in  which   it  vibrates.     If  the  <PA/ 

pendulum  be  raised  to  E  it  will  return  to  F,  if  it 
be  raised  to  C  it  will  return  to  D,  in  nearly  the  D 
same  length  of  time,  because  that,  in  proportion  ^ 
as  the  arc  is  more  extended,  the  steeper  will  be 
its   beginnings  anu  endings,  and,  therefore,  the  more  rapidly 
will  it  fall.* 


*  When  a  pendulum  is  raised  from  a  perpendicular  position,  its  weight 
will  cause  it  to  fall,  and,  in  the  act  of  falling,  it  acquires  a  degree  of  motion 
which  impels  it  to  a  height  beyond  the  perpendicular  almost  as  great  *is 
that  to  which  it  was  raised.  Its  motion  being  thus  spent,  gravity  again 
acts  upon  it  to  bring  it  to  its  original  perpendicular  position,  and  it  again 
acquires  an 'impetus  in  falling  which  carries  it  nearly  as  high  on  the  oppo- 
site side.  It  thus  continues  to  swing  backwards  and  forwards,  until  the 
resistance  of  the  air  wholly  arrests  its  motion. 

It  will  be  understood  that  gravity  affects  every  part  of  the  length  of  the 
pendulum.  A  ball  or  flattened  weight  is  attached  to  the  lower  end  of  the 
pendulum  to  concentrate  the  effects  of  gravity  in  a  single  point. 

In  the  construction  of  clocks,  an  apparatus  connected  with  the  weight  or 
the  spring  is  made  to  act  on  the  pendulum  with  such  a  force  as  to  enable  it 
to  overcome  the  resistance  of  the  air,  and  keep  up  a  continued  motion. 

f  It  has  already  been  stated  that  a  body  takes  the  same  time  in  rising 
und  falling  when  projected  upwards.  Gravity  brings  the  pendulum  down, 
arid  inertia  causes  it  to  continue  Its  motion  upwards. 

The  length  of  the  arc  in  which  a  pendulum   oscillates  is  called  its 


1052 


NATURAL    PHILOSOPHY 


On  what  does        379.  The  time  occupied  in  the  vibration  oi 

££££*  a  Pendulum  dePends  UP™  its  length.     The 
a  pendulum       longer  the  pendulum,  the  slower  are  its  vi- 
brations.* 


depend? 


What  is  the  .  °f    a   pendulum    which 

length  of  a       vibrates  sixty  times  in  a  minute  (or,  in  other 
words>  which  Crates  seconds)  is  about  thirty- 
inches.     But   in   different  parts  of  the 
iengtn  must  be  varied. 


'  to  vibrate  sec°nds  at  the 

seconds,  a  pendulum  equator,  must  be  shorter  than  one  which 
vibrates  seconds  at  the    oles- 


How  is  a  clock  381.  A  clock  is  regulated  by  lengthening 
regulated?  or  shortening  the  pendulum.  By  lengthening 
the  pendulum,  the  clock  is  made  to  go  slower  ;  by  shortening 
it,  it  will  go  faster.  J 

*  The  weight  of  the  ball  at  the  end  of  a  pendulum  does  not  affect  the 
duration  of  its  oscillations. 

t  The  equatorial  diameter  of  the  earth  exceeds  the  polar  diameter  by 
about  twenty-six  miles  ;  consequently  the  poles  must  be  nearer  to  the  centre 
of  the  earth's  attraction  than  the  equator,  and  gravity  must  also  operate 
with  greater  force  at  the  poles  than  at  the  equator.  Hence,  also,  the  length 
of  a  pendulum,  to  vibrate  in  any  given  time,  must  vary  with  the  latitude 
of  the  place. 

j:  The  pendulum  of  a  clock  is  made  longer  or  shorter  by  means  of  a  scre-w 
beneath  the  weight  or  ball  of  the  pendulum.  The  clock  itself  is  nothing 
more  than  a  pendulum  connected"  with  wheel-work,  so  as  to  record  the 
number  of  vibrations.  A  weight  is  attached  in  order  to  counteract  the 
retarding  effect  of  friction  and  the  resistance  of  the  air.  The  wheels  sh^w 
how  many  swings  or  beats  of  the  pendulum  have  taken  place  in  a  given 
time,  because  at  every  beat  the  tooth  of  a  wheel  is  allowed  to  pass.  Now, 
if  this  wheel  have  sixty  teeth,  it  will  turn  round  once  in  sixty  vibrations 
of  the  pendulum,  or  in  sixty  seconds  ;  and  a  hand,  fixed  on  the  axis  of  the 
wheel  projecting  through  the  dial-plate,  will  be  the  second-hand  of  the 
clock.  Other  wheels  are  so  connected  with  the  first,  and  the  number  of 
teeth  in  them  is  so  proportioned,  that  the  second  wheel  turns  -sixty  times 
slower  than  the  first,  and  to  this  is  attached  the  minute-hand  ;  and  the' 
third  wheel,  moving  twelve  times  slower  than  the  second,  carries  the  hour- 
baud.  On  account  of  the  expansion  of  the  pendulum  by  heat,  and  its  con- 
traction by  cold,  clocks  will  go  slower  in  summer  than  in  winter, 
the  pendulum  is  thereby  lengthened  at  that  season, 


REGULATORS   OF   MOTION".  103 

In  what  pro-  382.   The  lengths   of  pendulums  are  to 

portion  are  the     eadl  other  ag  the  re  of  the  time  ()f  tlieir 

l@7l(Jv/1S  OT 

pendulums  f       vibration. 

383.  According  to  this  law,  a  pendulum,  to  vibrate  once  in  two 
seconds,  must  be  four  times  as  long  as  one  that  vibrates  once  in  one 
second  ;  to  vibrate  once  in  three  seconds,  it  must  be  nine  times  as 
long  ;  to  vibrate  once  in  four  seconds,  it  must  be  sixteen  times  as 
long ;  once  in  five  seconds,  twenty-five  times  as  long,  &c. 

The  seconds  employed  in  the  vibrations  being 

1,  2,  3,  4,  5,  6,  7,  8,  9, 
the  length  of  the  pendulums  would  be  as 

1,4,9,16,25,36,49,64,81. 

A  pendulum,  therefore,  to*  vibrate  once  in  five  seconds,  must  be 
over  eighty  feet  in  length. 

384.  As  the  oscillations  of  a  pendulum  are  dependent  upon  gra- 
vitation, the  instrument  becomes  useful  in  ascertaining  the  force  of 
gravity  at  different  distances  from  the  centre  of  the  earth. 

385.  It  has  already  been  stated  that  the  centrifugal  force  at  the 
equator  is  greater  than  in  those  parts  of  the  earth  which  are  near 
the  poles.     As  the  centrifugal  force   operates  in  opposition  to  that 
of  gravity,  it  follows  that  the  pendulum  must  also  be  affected  by 
it ;  and  this  affords  additional  reason  why  a  pendulum,  to  vibrate 
seconds  at  the  equator,  must  be  shorter  than  one  at  the  poles.     It 
has  been  estimated  that,  if  the  revolution  of  the  earth  around  its 
axis  were  seventeen  times  faster  than  it  is,  the  centrifugal  force  at 
the  equator  would  be  equal  to  the  force  of  gravity,  and,  conse- 
quently, neither  could  a  pendulum  vibrate,  nor  would  bodies  there 
have  any  weight. 

386.  As  every  part  of  a  pendulum-rod  tends  to  vibrate  in  a  dif- 
ferent time,  it  is  necessary  that  all  pendulums  should  have  a  weight 
attached  to  them,  which,  by  its  inertia,  shall  concentrate  the  attract- 
ive force  of  gravity. 

387.  Pendulums  are    subject  to  variation  in  warm  and  cold 
weather,  on  account  of  the  dilatation  and  contraction  of  the  mate- 
rials of  which  the  rod  is  composed,  by  heat  and  cold.     For  this 
reason,  the  same  pendulum  is  always  longer  in  summer  than  it  is 
in  winter ;  and  a  clock  will,  therefore,  always  be  slower  in  summer 
than  in  winter,  unless  some  means  are  employed  by  which  the 
effects  of  heat  and  cold  on  the  length  of  the  pendulum  can  be  coun- 
teracted.    This  is  sometimes  effected  in  what  is  called  the  gridiron 
pendulum,  by  combining  bars  or  rods  of  steel  and  brass,  and  in  the 
mercurial  pendulum,  by  enclosing  a  quantity  of  quicksilver  in  a 
tube  near  the  bottom  of  the  pendulum. 

388.  In  order  to  secure  a  continuous  motion  to  the  pendulum 
(or,  in  other  words,  to  keep  a  clock  in  motion),  it  is  necessary  that 
the  pendulum  should  hang  in  a  proper  position.     A  practised  ear 
can  easily  detect  any  error  in  this  respect  by  the  irregularity  in  the 


L04  NATURAL    PHILOSOPHY. 

ticking,  or  (as  it  is  called)  by  its  being  "  out  of  beat."1  To  remedy 
this  fault,  it  is  necessary  either  to  incline  the  clock  to  the  one  side 
or  the  other,  until  the  tickings  are  synchronous ;  or,  in  other  words, 
are  made  at  equal  intervals  of  time.  It  can  sometimes  be  done 
without  moving  the  clock,  by  slightly  bending  the  upper  appendage 
of  the  pendulum  in  such  a  manner  that  the  two  teeth,  or  pro- 
jections, shall  properly  articulate  with  the  escapement-wheel.  [-See 
No.  303.] 


Table  of  the  Lengths  of  Pendulums  to  vibrate  Seconds  in  different  latitude* 


Inches. 

Inches 

At  the  equator,      39. 
Lat.  10°  North,      39.01 

At  the  equator, 
Lat.  10°  South, 

39. 

39.02 

20        '          39.04 

20 

39.04 

30         '          39.07 

30 

39.07 

40        '          39.10 

40 

39.10 

50        '          39.13 

50 

39.13 

60        *          39.16 

60 

390.  The  observations  have  been  extended  but  little  further,  north 
or  south  of  the  equator.  Different  observers  have  arrived  at  different 
results  ;  probably  on  account  of  their  different  positions  in  relation 
to  the  level  of  the  sea  in  whicfi  the  observations  were  made.     In 
such  a  work  as  this,  a  table  of  this  kind,  without  pretending  to  ex- 
treme accuracy,  is  useful,  as  showing  that  theory  has  been  con- 
firmed by  observation. 

391 .  The  moving  power  of  a  clock  is  a  weight,  which,  being  wound 
up,  makes  a  constant  effort  to  descend,  and  is  prevented  by  a  small 
appendage  of  the  pendulum,  furnished  with  two  teeth,  or  projec- 
tions, which  the  vibrations  of  the  pendulum  cause  alternately  to 
fall  between  the  teeth  of  a  wheel  called  the  escapement-wheel. 
The  escapement-wheel  is  thus  permitted  to  turn  slowly,  one  tooth 
at  a  time,  as  the  pendulum  vibrates.     If  the  pendulum  with  its 
appendage  be  removed  from  the  clock,  the  weight  will  descend  very 
rapidly,  causing  all  the  wheels  to  revolve  with  great  velocity,  and 
the  clock  becomes  useless  as  a  time-piece. 

392.  The  moving  power  of  a  watch*  is  a  spring,  called  the  main- 
spring, which  being  tightly  wound  around  a  cetitral  pin,  or  axis,  its 
elasticity  makes  a  constant  effort  to  loosen.     This  power  is  commu- 
nicated to  a  balance-wheel,  acted  upon  by  a  liair-spring,  and  having 
an  escapement  similar  to  that  of  the  .;lock.    If  the  hair-spring,  with 
the  escapement,  be  removed,  the  main-spring,  being  unrestrained, 


*  A  watch  differs  from  a  clock  in  having  a  vibrating  wheel,  instead  of  i> 
pendulum.  This  wheel  is  moved  by  a  spring,  called  the  hair-spring.  Th« 
place  of  the  weight  is  supplied  by  another  larger  spring,  called  the  main- 
spring. 


REGULATORS    OF    MOTION. 


Ho 


wUl  cause  the  wheels  to  revolve  with  great  rapidity,  ana  the  ,va\  ^ 

also,  becomes  useless  as  a  time-piece.* 

What  is  a  Bat-        393.  THE  BATTERING  RAM. —  The   Batter'ng 
Ram  was  a  military  engine  of  great  power,  ined 
to  beat  down  the  walls  of  besieged  places. 

Explain  394.  Its  construction,  and  the  principle  on  which  it 
Hg>5°'  was  Corked,  may  be  understood  by  inspection  of  >  ig. 
58,  in  which  A  B  represents  a  large  beam,  heavily  loaded  /  ith 

Fig.  58. 


a  noad  of  iron,  A,  resembling  the  head  of  a  ram,  from  which  it 
takes  its  name.  The  beam  is  accurately  balanced,  and  sus- 
pended by  a  rope  or  chain  C,  hanging  from  another  beam,  sup- 
ported by  the  frame  D  E  F  Gr.  At  the  extreme  end  B,  ropes 
or  chains  were  attached,  by  which  it  could  be  drawn  upwards 
through  the  arc  of  a  circle,  like  a  pendulum.  The  frame  wah 
sometimes  mounted  on  wheels. 

395.  Battering  rams  were  frequently  from  fifty  to  a  hundred 
feet  in  length,  and,  moving  with  a  force  compounded  of  their 
weight  and  velocity,  were  almost  irresistible.! 

*  As  a  regulator  of  motion,  the  pendulum  of  the  clock  is  to  be  lengthened 
or  shortened,  and  the  hair-spring  of  a  watch  is  to  be  tightened  or  loosened. 
This  is  to  be  done  in  the  former  case  in  the  manner  already  explained  in  the 
text ;  in  the  latter,  by  turning  what  is  called  the  regulator,  which  tightens 
or  loosens  the  hair-spring. 

t  The  ram  used  by  Demetrius  Poliorcetes  at  the  siege  of  Khodes  was 


106  NATURAL    PHILOSOPHY. 

096.     The  force  of  a  battering  rain  is  estimated  by  its  momentum 
ihat  is    its  weight  multiplied  by  its  velocity. 

397.      Questions  for  Solution. 

(1.)  Suppose  a  battering  rant  weighing  5760  Ibs.,  with  a  velocity  of  11 
feet  in  a  second,  could  penetrate  a  wall,  with  what  velocity  inudt  a  can- 
non-ball weighing  24  Ibs.  move  to  do  the  same  execution  1 

57GO  X  ll  =  63360  -:-  24  —  2640  feet,  or  one  half  of  a  mile  in  a  second 

(2.)  If  a  battering  ram  have  a  momentum  of  58,000  and  a  velocity  of  8. 
what  is  its  weight  '1  Ans.  7250 

(3.)  If  a  ram  have  a  weight  of  90,000  and  a  momentum  81,000,  what  is 
its  velocity  1  Ana.  .9 

(4.)  What  is  the  weight  of  %  ram  with  a  velocity  of  12  and  a  momentum 
60,000?  Ans.  5000. 

(5.)  Will  a  cannon-ball  of  9  Ibs.  and  a  velocity  of  3,000,  or  a  ram  with  a  weight  of 
15,000  and  a  velocity  of  2,  move  with  the  greater  force?  .  Ans.  The  ram, 

What  is  the      398.  THIS  GOVERNOR.— The  Governor  is  an 

Governor?      ...«,,  A    , 

ingenious  piecfc  of  mechanism,  constructed  on 

the  principle  of  the  centrifugal  force,  by  means  of  which 
the  supply  of  power  in  machinery  is  regulated.* 

Explain  399.  Fig.  59  represents  a  governor.  A  B  and 
rig.m.  ^  Q  are  ^WQ  ieverSj  or  arms,  loaded  with  heavy 

one  hundred  and  six  feet  long.  At  the  siege  of  Jerusalem  Vespasian  em- 
ployed a  ram  fifty  feet  long,  armed  with  an  iron  butt,  with  twenty-five  pro- 
jecting points,  two  feet  apart,  each  as  thick  as  the  body  of  a  man.  The 
counter  weight  at  the  hindmost  end  amounted  to  1075  cwt.,  and  1500  men 
were  required  to  work  the  machine. 

*  This  very  useful  appendage  to  machinery,  though  long  used  in  mills 
and  other  mechanical  arrangements,  owes  its  happy  adaptation  to  the  steam 
engine  to  the  ingenuity  of  Mr.  James  Watt. 

In  manufactures,  there  is  one  certain  and  determinate  velocity  with 
tfhich  the  machinery  should  be  moved,  and  which,  if  increased  or  dimin- 
ished, would  render  the  machine  unfit  to  perform  the  work  it  is  designed  to 
execute.  Now,  it  frequently  happens  that  the  resistance  is  increased  or 
diminished  by  some  of  the  machines  which  are  worked  being  stopped,  or 
others  put  on.  The  moving  power,  having  this  alteration  in  the  resistance, 
would  impart  a  greater  or  less  velocity  to  the  machinery,  were  it  not  for 
the  regulating  power  of  the  governor,  which  increases  or  diminishes  the 
supply  of  water  or  of  steam,  which  is  the  moving  power. 

13ut,  besides  the  alteration  in  the  resistance  just  noticed,  there  is,  also, 
frequently,  greater  changes  in  the  power.  The  heat  by  which  steam  is 
generated  cannot  always  be  perfectly  regulated.  At  times  it  may  afford  an 
excess,  and  at  other  times  too  little  expansive  power  to  the  steam.  Water, 
also,  is  subject  to  change  of  level,  and  to  consequent  alteration  as  a  moving 
power.  The  wind,  too,  which  impels  the  sails  of  a  wind-mill,  is  subject  to 
great  increase  and  diminution  To  remedy  all  these  inconveniences  is  th< 
duty  assigned  to  the  governor. 


itEGULATO-RS   (F    MOTION. 


107 


balls  at  their  extremities  B  and  C,  and 
suspended  by  a  joint  at  A  upon  the  ex- 
tremity of  a  revolving  shaft  AD.  A 
a  is  a  collar,  or  sliding  box,  connected 
with  the  levers  by  the  rods  It  a  and  c  a: 
with  joints  at  their  extremities.  When 
the  shaft  A  D  revolves  rapidly,  the  cen- 
trifugal force  of  the  balls  B  and  0  will 
cause  them  to  diverge  in  their  attempt  to 
fly  off,  and  thus  raise  the  collars,  by  means 
of  the  rods  b  a  and  c  a.  On  +he  con- 
trary, when  the  shaft  A  D  revolves  slowly,  the  weights  B  and 
C  will  fall  by  their  own  weight,  and  the  rods  b  a  and  c  a  will 
cause-the  collar  a  to  descend.  The  steam-valve  in  a  steam- 
engine,  or  the  sluice-gate  of  a  water-wheel,  being  connected 
with  the  collar  a,  the  supply  of  steam  or  water,  which  puts  the 
works  in  motion,  is  thus  regulated. 

What  is  the  ^®®'  ^ne  Main-spring  of  a  watch  consists  of  a 

Main-spring      long  ribbon  of  steel,  closely  coiled,  and  contained 
*f  a  watch?       in   a  round  box>     jt  ig  empi0ye(i  instead  of  a 

weight,  to  keep  up  the  motion. 

401.  As  the  spring,  when  closely  coiled,  exerts  a  stronger  force 
than  when  it  is  partly  loosened,  in  order  to  correct  this  inequality 
the  chain  through  which  it  acts  is  wound  upon  an  axis  surrounded 
by  a  spiral  groove  (called  a  fusee] ,  gradually  increasing  in  diameter 
from  the  top  to  the  bottom  ;  so  that,  in  proportion  as  the  strength 
9f  the  spring  is  diminished,  it  may  act  on  a  larger  lever,  or  a  larger 
wheel  and  axle. 

Explain  402.  Fig.  60  represents  a  spring  coiled  in  a  round  box 
Fig.  60.  A  B  is  the  fusee, 
surrounded  by  a  spiral  groove, 
on  which  the  chain  C  is  wound. 
When  the  watch  is  recently 
wound,  the  spring  is  in  the 
greatest  state  of  tension,  ana 
will,  therefore,  turn  the  fusee 
5 


Fig.  60. 


108  NATUKAL  PHILOSOPHY. 

by  the  smallest  groove,  on  tlie  principle  of  the  wheel  and 
axle.  As  the  spring  loses  its  force  by  being  partly  un- 
wound, it  acts  upon  the  larger  circles  of  the  fusee  ;  and 
the  want  of  strength  in  the  spring  is  compensated  by  the 
mechanical  aid  of  a  larger  wheel  and  axle  in  the  larger 
grooves.  By  this  means  the  spring  is  made  at  all  times  to 
exert  an  equal  power  upon  the  fusee.  The  motion  is  com- 
municated from  the  fusee  by  a  cogged  wheel,  which  turns 
with  the  fusee. 

Of  what  does  403.  HTDEOSTATICS.*  —  Hydrostatics  treats 
Hydrostatics  „  .,  .,  „  n  .  , 

treat  ?  °f  the  nature,  gravity  and  pressure  of  fluids. 

What  is  tTie  dif-  404.  Hydrostatics  is  generally  confined  to 
&£%%£  the  consideration  of  fluids  at  rest,  and  Hy- 


Hydrostatics  f      draulics  to  fluids  in  motion. 

What  is  a  405.  A  Fluid  is  a  substance  which  yields 

Fluid  f  fa  the  slightest  pressure,  and  the  particles  of 

which,  having  but  a  slight  degree  of  cohesion,  move  easily 
among  themselves.! 

*  The  suijects  of  Hydraulics  and  Hydrostatics  are  sometimes  descrioea 
under  the  general  name  of  Hydrodynamics.  The  three  terms  are  from  the 
Greek  language,  compounded  of  nJo»p  (hudor),  signifying  water,  and  Svrums 
(dunamis)  ,  force  or  power  ;  oraTtxog  (staticos),  standing,  and  uuXog  (aulos),  a 
tube  or  pipe.  Hence  Hydrodynamics  would  imply,  the  science  which  treats 
of  the  properties  and  relations  of  water  and  other  fluids,  whether  in  a  state 
of  motion  or  rest  ;  while  the  term  Hydrostatics  would  be  confined  to  the 
consideration  of  fluids  in  a  state  of  rest,  and  Hydraulics  to  fluids  in  motion 
through  tubes  or  channels,  natural  or  artificial. 

t  There  is  this  remarkable  difference  between  bodies  in  a  fluid  and 
bodies  in  a  solid  form,  namely,  that  every  particle  of  a  fluid  is  perfectly 
independent  of  every  other  particle.  They  do,  not  cohere  in  masses,  like 
the  particles  of  a  solid,  nor  do  they  repel  one  another,  as  is  the  case  with  the 
particles  composing  a  gas.  They  can  move  among  one  another  with  tho 
least  degree  of  friction,  and,  when  they  press  down  upon  one  another  ir 
virtue  of  their  own  weight,  the  downward  pressure  is  communicated  in  aU 
directions,  causing  a  pressure  upwards,  sideways,  and  in  every  possible 
manner  Herein  the  particles  of  a  fluid  differ  from  the  particles  of  a  solid, 
even  when  reduced  to  the  most  impalpable  powder  ;  and  this  it  is  which  con, 
ttitutes  fluidity,  namely,  the  power  of  transmitting  pressure  in  every  direction. 
and  that,  too,  with  the  least  degree  of  friction.  The  particles  whioh  compos* 
a  fluid  must  be  very  much  smaller  than  the  finest  gnan  of  OD  iiuyalt>.i')le 
pow  ier. 


HYDROSTATICS.  .  109 

Sow  does  a  406.   A  liquid  differs  from  a  gas  in  its  de- 

lfroma^or    ^Qe  of  compressibility  and  elasticity.    Gases 
\japor  ?  are  highly  compressible  and  elastic.    Liquids, 

on  the  contrary,  have  but  a  slight  degree  either  of  com- 
pressibility or  of  elasticity.* 

407.  Another  difference  between  a  liquid  and  a  gas  arises  from 
the  propensity  which  gases  have  to  expand  whenever  all  external 
pressure  is  removed.     Thus,  whenever  a  portion  of  air  or  gas  is 
removed  from  a  closed  vessel,  the  remaining  portion  will  expand, 
and,  in  a  rarer  state,  will  fill  the  whole  vessel.     Liquids,  on  the 
contrary,  will  not  expand  without  a  change  of  temperature.    Liquids 
also  have  a  slight  degree  of  cohesion,  in  virtue  of  which  the  particles 
will  form  themselves  into  drops ;  but  the  particles  of  gases  seem  to 
possess  the  opposite  quality  of  repulsion,  which  causes  them  to  ex- 
pand without  limit,  unless  confined  within  the  bounds  of  some  ves- 
sel, or  restricted  within  a  certain  bulk  by  external  pressure. 

408.  The  fluid  form  of  bodies  seems  to  be  in  great,  measure,  if 
not  wholly,  attributed  to  heat.     This  subtle  agent  insinuates  itfeelf 
between  the  particles  of  bodies,  and  forces  them  asunder.     Thus, 
for  instance,  water  divested  of  its  heat  becomes  ice,  which  is  a 
solid.     In  the  form  of  water  it  is  a  liquid,  having  but  in  a  very 
slight  degree  the  properties  either  of  compressibility  or  elasticity. 
An  additional  supply  of  heat  converts  it  into  steam,  endowed  with 
a  very  great  degree  both  of  elasticity  and  compressibility.     But,  so 
soon   as   steam   loses  its  heat,  it  is  again  converted  into  water. 
Again,  the  metals  become  liquid  when  raised  to  certain  tempera- 
tures, and  it  is  known  that  many,  and  supposed  that  all,  of  them 
would  be  volatilized  if  the  required  supply  of  heat  were  applied. 


*  The  celebrated  experiment  made  at  Florence,  many  years  ago,  to  test 
the  compressibility  of  water,  led  to  the  conclusion  that  water  is  wholly 
incompressible.  Later  experiments  have  proved  that  it  may  be  com 
pressed,  and  that  it  also  has  a  slight  degree  of  elasticity.  In  a  voyage  to 
the  West  Indies,  in  the  year  1839,  an  experiment  was  made,  at  Vne  sugges- 
tion of  the  author,  with  a  bottle  filled  with  fresh  water  from  the  tanks  on 
the  deck  of  the  Sea  Eagle.  It  was  hermetically  sealed,  and  let  down  to  the 
depth  of  about  seven  hundred  feet.  On  drawing  it  up,  the  bottle  was  still 
full,  but  the  water  was  brackish,  proving  that  the  pressure  at  that  great 
depth  had  forced  a  portion  of  the  deep  salt  water  into  the  bottle,  previously 
compressing  the  water  in  the  bottle  to  make  room  for  it.  As  it  rose  to  the 
surface,  its  elasticity  restored  it  to  its  normal  state  of  density. 

At  great  depths  in  the  sea  the  pressure  of  the  superincumbent  mass 
increases  the  density  by  compression,  and  it  has  been  calculated  that,  at  H 
depth  of  about  ninety  miles,  water  would  be  compressed  into  one-half  of  ite- 
volume,  and  at  a  depth  of  360  miles  its  density  would  be  nearly  equal  t»< 
that  of  mercury.  Under  a  pressure  of  15,000  Ibs.  to  a  square  inch,  .Mr. 
Perkins,  of  Newburyport,  subsequently  of  London,  has  sh:wn  that  witer  ia 
reduced  in  bulk  one  part  iu  twenty-four. 


110  NATUKAL    PHILOSOPHY. 

The  science  of  Geology  furnishes  sufficient  reasons  for  believing 
that  all  known  substances  were  once  not  only  in  the  liquid  form, 
but  also  previously  existed  in  the  form  of  gag.* 

How  do  fluids  409.  GRAVITATION  OF  FLUIDS.  —  Fluids  gravi- 
gramtatef  ^a^e  jn  a  more  perfect  manner  than  solids,  oc 
account  of  their  want  of  cohesiv  e  attraction.  The  particles  of  a 
solid  body  cohere  so  strongly  that,  when  the  centre  of  gravity 
is  supported,  the  whole  mass  vill  be  supported.  But  every 
particle  of  a  fluid  gravitates  independently  of  every  other  par- 
ticle. 

yy,  410.  On  account  of  the  independent  gravita- 

fluids  be  tion  and  want  of  cohesion  of  the  particles  of  a 

moulded  into     flui(j>  they  cannot  be  formed  into  figures,  nor  pre- 
served in  heaps.     Every  particle  makes  an  effort 
to  descend,  and  to  preserve  what  is  called  the  level  or  equi- 
librium. 


What  is  the  ^11.  The  level  or  equilibrium  of  fluids  ia 
equilibrium  of  the  tendency  of  the  particles  so  to  arrange 
themselves  that  every  part  of  the  surfao 
shall  be  equally  distant  from  the  centre  of  the  earth  ;  that 
is.  from  the  point  towards  which  gravity  tends. 

What  is  the          412.  Hence  the  surface  of  all  fluids,  when  in  a 

Surface*  of€all     state  of  rest)'  Partakes  tlie  spherical  form  of  the 
fluids  ?  earth. 

413.  For  the  same  reason,  a  fluid  immediately  conforms  itself  tc> 
the  shape  of  the  vessel  in  which  it  is  contained.  The  particles  of  a 
solid  body  being  united  by  cohesive  attraction,  if  any  one  of  them 
be  supported  it  will  uphold  those  also  with  which  it  is  united. 
But,  when  any  particle  of  a  fluid  is  unsupported,  it  is  attracted 
down  to  the  level  of  the  surface  of  the  fluid  ;  and  the  readiness  with 
which  fluids  yield  to  the  slightest  pressure  will  enable  the  particle, 

>y  its  own  weight,  to  penetrate  the  surface  of  the  fluid,  and  mis 

jrith  it. 


*  The  science  of  Chemistry  unfolds  the  fact  that  all  the  great  changes  in  the 
constitution  of  bodies  are  accompanied  by  the  exhibition  of  heat  either  in  a  free 
or  latent  condition. 


*     HYDROSTATICS.  11 J 

mat  is  Ca-  414.  OAPILLAKY  ATTRACTION. —  Capillary 
pMaryAttrac-  Attraction  is  that  attraction  which  causes 
What  are  Ca-  fluids  to  ascend  above  their  level  in  capillary 
pillary  Tubes  ?  tubes.  Capillary  *  tubes  are  tubes  with  very 
fine  bore. 

415.  This  kind  of  attraction  exhibits  itself  not  only  in  tubes,  but 
also  between  surfaces  which  are  very  near  together.     This  may  be 
beautifully  illustrated   by   the    following  experiment.     Take   two 
pieces  of  flat  glass,  and,  having  previously  wet  them,  separate  their 
edges  on  one  side  by  a  thin  strip  of  wood,  card  or  other  material ; 
tie  them  together,  and  partly  immerse   them   perpendicularly  in 
colored  water.     The  water  will  then  rise  the  highest  on  that  side 
vhere  the  edges  of  the  glass  meet,  forming -a  beautiful  curve  down- 
wards towards  the  edges  which  are  separated  by  the  card. 

416.  Immeree  a  number  of  tubes  with  fine  bores  in  a  glass  of 
colored  water,  and  the  water  will  rise  above  its  equilibrium  in  all, 
but  highest  in  the  tube  with  the  finest  bore. 

417.  The  cause  of  this  seems  to  be  nothing  more  than  the  ordi- 
nary attraction  of  the  particles  of  matter  for  each  other.     The  sides 
of  a  small  oiifice  are  so  near  to  each  other  as  to  attract  the  particles 
of  the  fluid  on  their  opposite  sides,  and,  as  all  attraction  is  strongest 
in  the  direction  of  the  greatest  quantity  of  matter,  the  water  is 
raised  upwards,  or  in  the  direction  of  the  length  of  the  tube.     On 
the  outside  of  the  tube,  the  opposite  surfaces  cannot  act  on  the 
same  column  of  water,  and,  therefore,  the  influence  of  attraction  is 
here  imperceptible  in  raising  the  fluid. 

418.  All  porous  substances,  such  as  sponge,  bread,  linen,  sugar, 
&c.,  may  be  considered  as  collections  of  capillary  tubes;  and,  for 
this  reason,  water  and  other  liquids  will  rise  in  them  when  they  are 
partly  immersed. 

419.  It  is  on  the  same  principle  that  the  wick  of  a  lamp  will 
carry  up  the  oil  to  supply  the  flame,  although  the  flame  is  several 
inches  above  the  level  of  the  oil.f     If  the  end  of  a  towel  happen  to 

*  The  ^ord  capillary  is  derived  from  the  Latin  word  capilla  (hair),  and  it 
Is  applied  to  this  kind  of  attraction  because  it  is  exhibited  most  prominently 
In  tubes  the  borex  of  which  are  as  fine  as  a  hair,  and  hence  called  capillary 
kabes. 

t  The  reason  why  well-filled  lamps  will  sometimes  fail  to  give  light  is, 
lhat  the  wick  is  too  large  for  its  tube,  and,  being  thus  compressed,  the 
japillary  attraction  is  impeded  by  the  compression.  The  remedy  is  to 
reduce  the  size  of  the  wick.  Another  cause,  also,  that  prevents  a  clear 
light,  is  that  the  flame  is  too  far  from  the  surface  of  the  oil.  As  capillary 
ittraction  acts  only  at  short  distances,  the  surface  of  the  oil  should  always 
fc«  within  a  short  distance  of  the  flame.  But  another  reason,  which  requires 
particular  attention,  is,  that  all  kinds  of  oil  usually  employed  for  lamps 
contain  a  glutinous  matter,  of  which  no  treatment  can  wholly  divest  them. 
This  matter  fills  the  pores  or  capillary  tubes  of  the  wick,  arid  prevents  the 


1-  NATURAL    PHILOSOPHY. 

be  left  in  a  basin  of  water,  it  will  empty  the  basin  of  its  contents 
On  the  same  principle,  when  a  dry  wedge  of  wood  is  diiven.  into 
the  crevice  of  a  rock,  as  the  rain  falls  upon  it,  it  will  absorb  the 
water,  swell,  and  sometimes  split  the  rock.  In  this  manner  mill- 
stone quarries  are  worked  in  Germany. 

420.  ENDOSMOSE  AND   EXOSMOSE. — In   addition   to  the   capillary 
attraction  just  noticed  as  peculiar  to  fluids,  another  may  be  men- 
tioned, as  yet  but  imperfectly  understood,  which  seems  to  be  due 
partly  to  capillary  and  partly  to  chemical  attraction,  known  under 
the  names  endosmose  and  exosmose.*     These  phenomena  are  mani- 
fested in  the  transmission  of  thin  fluids,  vapor  and  gaseous  matter, 
through  membranes  and  porous  substances.     The  ascent  of  the  sap 
in  vegetable,  and  the  absorption  of  nutritive  matter  by  the  organs 
of  animal  life,  are  to  be  ascribed  to  these  causes. 

421.  When  two  liquids  of  different  densities  are  separated  by  a 
membranous  substance  or  by  porcelain  unglazed,  endosmose  will 
carry  a  current  inwards,  and  exosmose  will  force  one  outwards,  thup 
causing  a  partial  mixture  of  the  fluids. 

.  422.  Eocperimsxt. —  Take  a  glass  tube,  and,  tying  a  piece  of  bladder  01 
clean  leather  over  one  end  for  a  bottom,  put  some  sugar  into  it,  and  having 
poured  a  little  water  on  the  sugar,  let  it  stand  a  few  hours  in  a  tumbler,  of 
water.  It  will  then  be  found  that  the  water  has  risen  in  the  tube  through 
the  membranous  substance.  This  is  due  to  endosmose.  If  allowed  to  stand 
several  days,  the  liquid  will  rise  several  feet. 

If  the  experiment  be  reversed,  and  pure  water  be  put  into  the  tube,  and 
the  moistened  sugar  into  the  tumbler,  the  tube  will  be  emptied  by  exosmose. 

423.  The  liquid  that  has  the  less  density  will  generally  pass  to  the 
denser  liquid  and  dilute  it. 

What  peculi-  424.  GRAVITATION  OF  FLUIDS  or  DIFFERENT 
arity  is  there  DENSITIES.  —  When  solid  bodies  are  placed  one 
ltation  of'fluids  a^ove  another,  they  will  remain  in  the  position  in 
of  different  which  they  are  placed  so  long  as  their  respective 
densities?  centres  of  gravity  are  supported,  without  regard 
to  their  specific  gravity.  With  fluids  the  case  is  different. 


ascent  of  the  oil  to  feed  the  flame.  For  this  reason,  the  wicks  of  lamps 
should  be  often  renewed.  A  wick  that  has  been  long  standing  in  a  lamp 
will  rarely  afford  a  clear  and  bright  light.  Another  thing  to  be  noticed  by 
those  who  wish  the  lamp  to  perform  its  duty  in  the  best  possible  manner 
is,  that  the  wick  be  not  of  such  size  as,  by  its  length,  as  well  as  its  thickness, 
to  fill  the  cup,  and  thereby  leave  no  room  for  the  oil.  It  must  also  be 
remembered  that,  although  the  wick  when  first  adjusted  may  be  of  the 
proper  size,  the  glutinous  matter  of  the  oil,  filling  its  capillary  tubes,  causes 
the  wick  to  swell,  and  thereby  become  too  large  for  the  tube,  producing  the 
Fame  difficulty  as  has  already  been  noticed  in  cases  where  the  wick  is  too 
large  to  allow  the  free  operation  of  capillary  attraction, 

*  Endosmose,  from  evdov,  within,  and  ua^og,  impulsion      Exosmose,  from 
i,'?,  uulwai  d,  aud  uj«juo{,  impulsion 


•  HYDKOSTATICS. 

Fluids  of  different  specific  gravity  will  arrange  themselves  in 
the  order  of  their  density,  each  preserving  its  own  equilibrium. 

425.  Thus,  if  a  quantity  of  mercury,  water,  oil  and  air,  be  put 
into  the  same  vessel,  they  will  arrange  themselves  in  the  order  of 
their  specific  gravity.  The  mercury  will  sink  to  the  bottom,  the 
water  will  stand  above  the  mercury,  the  oil  above  the  water,  and 
the  air  above  the  oil ;  and  the  surface  of  each  fluid  will  partake  of 
the  sphericaJ  form  of  the  earth,  to  which  they  all  respectively 
gravitate. 

What  is  a  Spirit  426.  A  Water  or  Spirit  Level  is  an  in- 
Level,  or  Water  strument  constructed  on  the  principle  of  the 
equilibrium  of  fluids..  It  consists  of  a  glass 
tube,  partly  filled  with  water,  and  closed  at  both  ends. 
When  the  tube  is  not  perfectly  horizontal, —  that  is,  if  one 
end  of  the  tube  be  lower  than  the  other, —  the  water  will 
run  to  the  lower  end.  By  this  means  the  level  of  any  line 
to  which  the  instrument  is  applied  may  be  ascertained. 

427.  Fig.  61  represents  a  Water  Level.  A  B  is  a 
Fig  61*  S^ass  *u^e  partly  filled  *ith  water. 
C  is  a  bubble  of  air  occupying  the 
space  not  filled  by  the  water.  When  both 
ends  of  the  tube  are  on  a  level,  the  air-bubble 
will  remain  in  the  centre  of  the  tube ;  but,  if  either  end  of  the 
tube  be  depressed,  the  water  will  descend  and  the  air-bubble 
will  rise.  The  glass  tube,  when  used,  is  generally  set  in  a  woode^ 
or  a  brass  box.  It  is  an  instrument  much  used  by  carpenten 
masons,  surveyors,  &c. 

.[N.  B.  The  tube  is  generally  filled  with  spirit,  instead  of  water,  o» 
account  of  the  danger  that  the  water  will  freeze  and  burst  the  gliss.  Henct 
the  instrument  is  called  indifferently  the  Spirit  Level  or  the  Water  Level.] 

428.  EFFECT  OF  THE  PECULIAR  GRAVITATION 
Why  do  falling  _.  . 

fluids    do    less    OF  -FLUIDS. —  bond  bodies  gravitate   in  masses, 

damage      than    their  parts    being   so   connected  as   to    form  a 

whole,  and  their  weight  may  be  regarded  as 

concentrated  in  a  point,  called  the  centre  of  gravity;  while  each 


114  NATUKAL   PHILOSOPHY.   - 

particle  of  a  fluid  may  be  considered  as  a  separate  mass,  gravi- 
tating independently. 

It  is  for  this  reason  that  a  body  of  water,  in  falling,  does  less 
injury  than  a  solid  body  of  the  same  weight.  But  if  the  water  be 
converted  into  ice,  the  particles  losing  their  fluid  form,  and  being 
united  by  cohesive  attraction,  gravitate  unitedly  in  one  mass. 

In  what  direc-  4-29.  PEESSUEE  OF  FLUIDS. — Fluids  not 
^±SL*  only  Press  downwards  like  solids,  but  also 
of  their  weight  f  upwards,  sidewise,*  and  in  every  direction. 
(See  Appendix,  par.  1418.) 

430.  So  long  as  the  equality  of  pressure  is  undisturbed,  every 
particle  will  remain  at  rosv.  If  the  fluid  be  disturbed  by  agitating 
it,  the  equality  of  pressuie  will  be  disturbed,  and  the  fluid  will  not 
rest  until  the  equilibrium  io  restored. 

TT                 ffo  431.  The   downward   pressure   of   fluids   is 

downward,   lat-  shown  by  making  an  aperture  in  the  bottom  of 

eral     and    up-  a  yessei  Of  watcr.     Every  particle  of  the  fluid 

ward     pressure  .           .                         .„           ,                1,1 

qf  fluids  shown  ?  above  the  aperture  will  run  downwards  through 

the  opening. 

432.  The  lateral  pressure  is  shown  by  making  the  aperture 
at  the  side  of  the  vessel.     The  fluid  will  then  escape  through 
the  aperture  at  the  side. 

433.  The  upward  pressure  is  shown  by  taking  a  glass  tube, 
open  at  both  ends,  inserting  a  cork  in  one  end  (or  stopping  it 
with  the  finger),  and  immersing  the  other  in  the  water.     The 
water  will  not  rise  in  the  tube.     But  the  moment  the  cork  is 
taken  out  (or  the  finger  removed),  the  fluid  will  rise  in  the  tube 
to  a  level  with  the  surrounding  water. 

Pig.  62.  *  If  the  particles  of  fluids  were   arranged  in 

Fig.  63.  regular  columns,  as  in  Fig.  62,  there  would  be 
no  lateral  pressure  ;  for  when  one  particle  is  per- 
pendicularly above  the  other,  it  can  press  only 
downwards.  But,  if  the  particles  be  arranged  as 
in  Fig.  63,  where  a  particle  presses  between  tw« 
particles  beneath,  these  last  must  suffer  a  lateral  pressure.  In  whatever 
manner  the  particles  are  arranged,  if  they  be  globular,  as  is  supposed,  there 
muht  be  spaces  between  them  \See  Fig.  I,  page  22.] 


HYDROSTATICS. 


115 


What  is  the  '  Pressure  °    a    u        s  in  PrOP°r" 

law  of  fluid  tion  to  the  perpendicular  distance  from  the 
surface;  that  is,  the  deeper  the  fluid,  the 
greater  will  be  the  pressure.  This  pressure  is  exerted  in 
every  direction,  so  that  all  the  parts  at  the  same  depth 
press  each  other  with  equal  force.  (See  par.  1423.) 

435.  A  bladder,  filled  with  air,  being  immersed  in  water,  will 
te  contracted  in  size,  on  account  of  the  pressure  of  the  water  in  all 
Hrectiong ;  and  the  doeper  it  is  immersed,  the  more  will  it  be  con- 
tacted.* 

436.  An  empty  bottle,  being  corked,  and,  by  means  of  a  weight, 
iet  down  to  a  certain  depth  in  the  sea,  will  either  be  broken  by  the 
pressure,  or  t>tO  cork  will  be  driven  into  it,  and  the  bottle  be  filled 
with  wetrr.     This  will  take  place  even  if  the  cork  be  secured  with 
wire  and  peeled.    But  a  bottle  filled  with  water,  or  any  other  liquid, 
may  be  iet  down  to  any  depth  without  damage,  because,  in  this 
case,  the  internal  pressure  is  equal  to  the  external. f 


*  T'ae  weight  of  a  cubic  inch  of  water  at  the  temperature  of  62o  of  Fah- 
lonheit's  thermometer  is  36066  millionths  of  a  pound  avoirdupois.  The 
pr&asure  of  a  column  of  water  of  the  height  of  one  foot  will  therefore  be 
twelve  times  this  quantity,  or  .4328  (making  allowance  for  the  repeating 
decimal),  and  the  pressure  upon  a  square  foot  by  a  column  one  foot  high 
will  be  found  by  multiplying  this  last  quantity  by  144,  the  number  of 
square  inches  in  a  square  foot,  and  is  therefore  62.3332 

Hence,  at  the  depth  of 

Ibs.  Ibs. 


1  foot 

2  feet 

3  " 

4  " 

5  " 

6  " 

7  " 

8  " 

9  " 

10  " 
100" 


the  pressure  on  a  square  in-ih  is 


.4328,  on  a  square  foot,     62.3232 

.8656,  124.6464 

1.2984,  186.9696 

1.7312,  249.2928 

2.1640,  311.6160 

2.5968,  373.9392 

3.0296,  436.2624 

3.4624,  •               498.5856 

3.8952,  '        560.9088 

4.3280,  «  «        623.2320 

43.2800,  "  "      6232.3200 


Fiom  this  table,  the  pressure  on  at»y  ..-u^face  at  any  depth  may  easily  be 
found. 

It  will  thus  be  seen  that  there  is  a  certain  limit  beyond  which  divers 
cannot  plunge  with  impunity,  nor  fishes  of  any  kind  live.  Wood  that  has 
been  sunk  to  great  depths  in  the  sea  will  have  its  pores  so  filled  with 
water,  and  its  specific  gravity  so  increased,  that  it  will  no  longer  float. 

f  "  Experiments  at  Sea  —  We  are  indebted  to  a  friend,  who  has  just  arrived 
from  Europe,  says  the  Baltimore  Gazette,  for  the  fol'owing  experiments 
made  on  board  the  Charlemagne  : 

•-  26th  of  September,  1836,  tko  weather  being  calm,  I    corked  an  cuiptj 

5* 


lit)  NATURAL   PHILOSOPHY. 


437.  Questions  for  Solution. 

(1.)  What  pressure  is  sustained  by  the  body  of  a  fi**h  having  a  surface  of 
i>  square  feet  at  the  depth  of  150  feet  1  -4ns.  8-4136.32  Ib. 

(2.)  What  is  the  pressure  on  a  square  yard  of  the  banks  of  a  canal,  at  the 
depth  of  four  feet  \  .  Ana.  ii243.tis52  tt>. 

(3.)  What  pressure  is  exerted  on  the  body  of  a  man,  at  the  depth  of 
30  feet,  supposing  the  surface  of  his  body  to  be  2j  sq.  yd.1  Ans.  4206S.1<5#>. 

(4.)  Suppose  a  whale  to  be  at  the  depth  of  200  feet,  and  that  his  body 
.presents  a  surface  of  150  yards.  What  is  the  pressure  ?  Ans.  16827264  Ib. 

(5.)  How  deep  may  a  glass  vessel  containing  18  inches  of  square  surface 
be  sunk  without  being  broken,  supposing  it  capable  of  resisting  an  equal 
pressure  of  1500  lbs.1  Ans.  192.54/5.  + 

(6.)  What  is  the  pressure  sustained  on  the  sides  of  a  cubical  water-tight 
box  at  the  depth  of  150  feet  below  the  surface,  supposing  the  box  to  rest  on 
'he  bed  of  the  sea,  and  each  side  to  be  8  feet  square?  An*.  299151.36/6. 

(7.)  How  deep  can  a  glass  vessel  be  sunk  without  breaking,  srjpposing 
that  it  be  capable  of  resisting  a  pressure  of  200  pounds  on  a  square  inch  \ 

Ans.  462.1  /t + 

438.  The  lateral  pressure  of  a  fluid  proceeds 

14  hat  causes  the    entirely  from  the  pressure    downwards,   or.   in 
lateral  pressure  ,  .  _ 

of  fluids  ?  other   words,   from   the   weight   or  the   liquid 

above ;  consequently,  the  lower  an  orifice  is 
made  in  a  vessel  containing  water  or  any  other  liquid,  the 
greater  will  be  the  force  and  velocity  with  which  the  liquid  will 
rush  out. 


wine-bottle,  and  tied  a  piece  of  linen  over  the  cork  ;  1  then  sank  it  intf 
the  sea  six  hundred  feet  ;  when  drawn  immediately  up  again,  the  cork  waf 
inside,  the  linen  lemained  as  it  was  placed,  and  the  bottle  was  filled  with 
water. 

*'  I  next  made  a  noose  of  strong  twine  around  the  bottom  of  the  cork, 
which  I  forced  into  the  empty  bottle,  lashed  the  twine  securely  to  the  necb 
i)l'  the  bottle,  and  sank  the  bottle  six  hundred  feet.  Upon  drawing  it  up 
immediately,  the  cork  was  found  inside,  having  forced  its  way  by  the  twine, 
and  in  so  doing  had  broken  itself  in  two  pieces  ;  the  bottle  was  filled  with 
water. 

"  I  then  made  a  stopper  of  white  pine,  long  enough  to  reach  to  the  bot- 
tom of  the  bottle;  after  forcing  this  stopper  into  the  bottle,  I  cut  it  ofif  about 
half  an  inch  above  the  top  of  the  bottle,  and  drove  two  wedges,  of  the  same 
wood,  into  the  stopper.  I  sank  it  six  hundred  feet,  and  upon  drawing  it 
up  immediately  the  stopper  remained  as  I  place'd  it,  and  there  was  about 
a  gill  of  water  in  the  bottle,  which  remained  unbroken.  The  water  must 
have  forced  its  way  through  the  pores  of  the  wooden  stopper,  although 
wedged  as  aforesaid  ;  and  had  the  bottle  remained  sunk  long  enough,  there 
is  no  doubt  that  it  would  have  been  filled  with  water."  [See  also  note  on 
page  109.] 

It  is  the  opinion  of  some  philosophers  that  the  pressure  at  very  great 
depths  of  the  sea  is  so  great  that  the  water  is  condensed  into  a  solid  state  j 
*nd  that  at  or  near  the  centre  of  the  earth,  if  the  fluid  could  extend  so 
deeply,  this  pressure  would  convert  the  whole  into  a  solid  mass  of  firo. 


HYDROSTATICS. 


11? 


Fig.  64. 


489    Fig.  64  represents  a  vessel  of  water,  with  ori 
fices  at  the  side  at  different  dis- 
tances   from    the    surface.      The 
different  curves  in  the  figure,  described  by 
the  liquid  in  running  out  of  the  vessel,  show 
the  action  of  gravity,  and  the  effects  pro- 
duced by  the  force  of  the  pressure   on  the 
liquid  at  different  depths.     At  A  the  press- 
ure is  the  least,  because  there  is  less  weight  of  fluid  abo\e. 
At  B  and  C  the  fluid  is  driven  outwards  by  the  weight  of  that 
portion  above,  and  the  force  will  be  strongest  at  C. 

440.  As  the  lateral  pressure  arises  solely 
from  the  downward  pressure,  it  is  not  affected 
by  the  width  nor  the  length  of  the  vessel  in 


What  effect  has 
the  length  and 
the  width  of  a 
body  of  fluid 


upon  its  lateral    which  it  is  contained,  but  merely  by  its  depth ; 
pressure  ?  ^  ag  everv  particle  acts  independently  of  the 

rest,  it  is  only  the  column  of  particles  above  the  orifice  that  cap 
weigh  upon  and  press  out  the  water. 

To  what  is  the  •  441.  The  lateral  pressure  on  one  side  of  a 
lateral  pressure  cubical  vessel  will  be  equal  only  to  half  of  the 
pressure  downwards ;  for  every  particle  at  the 
bottom  of  a  vessel  is  pressed  upon  by  a  column  of  the  whole  depth 
of  the  fluid,  while  the  lateral  pressure  diminishes  from  the  bottom 
upwards  to  the  surface,  where  the  particles  have  no  pressure. 
What  causes  the  442.  The  upward  pressure  of  fluids,  althougl 
upward  pressure  apparently  in  opposition  to  the  principles  of 
gravity,  is  but  a  necessary  consequence  of  the 
operation  of  that  principle ;  or,  in  other  words,  the  pressure 
upwards,  as  well  as  the  pressure  downwards,  is  caused  by  gravity. 

443.  When  water  is  poured  into  a  vessel  with  a 
8Pout  (like  a  tea-Pot>  f°r  instance),  the  water  rises  in 
the  spout  to  a  level  with  that  in  the  body  of  the  ves- 
sel.    The  particles  of  water   at  the  bottom  of  the  vessel  are 
pressed  upon  by  the  particles  above  them,  and  to  tins  pressure 
they  will  yield,  if  there  is  any  mode  of  making  way  for   the 


118  NATURAL    PHILOSOPHY. 

particles  above  them.     As  they  cannot  descend  R 

through  the  bottom  of  the  vessel,  they  will 
change  their  direction  and  rise  in  the  spout. 
Fig.  65  represents  a  tea-pot,  and  the  columns 
of  balls  represent  the  particles  of  water  magni- 
fied. From  an  inspection  of  the  figure,  it  appears  that  the  par- 
tide  numbered  1,  at  the  bottom,  will  be  pressed  laterally  by  the 
particle  numbered  2,  and  by  this  pressure  forced  into  the  spout, 
where,  meeting  with  the  particle  3,  it  presses  it  upwards,  ana 
this  pressure  will  be  continued  from  3  to  4,  from  4  to  5,  and  so 
on,  till  the  water  in  the  spout  has  risen  to  a  level  with  that  in 
the  body  of  the  vessel.  If  water  be  poured  into  the  spout,  the 
water  will  rise  in  the  same  manner  in  the  body  of  the  vessel , 
from  which  it  appears  that  the  force  of  pressure 
dePends  entirely  on  the  height,  and  not  on  the 
ture.  length  or  breadth,  of  the  column  of  fluid.  [Sen 

No.  434.] 

444,  Any  quantity  of  fluid,  however  small, 

What  is  the    m       j^  ma(je  ^0  kalance  any  other  quantity 
Hydrostatic          *  J  ^  J 

Paradox  ?      however  large.     This  is  what  is  called  the  Hy- 
drostatic Paradox.* 

Explain  445.  The  principle  of  what  is  called  the  hydro- 
Fig.  66.  static  paradox  is  illustrated  by  the  hydrostatic  bellows 
represented  in  Fig.  66  A  B  is  a  long  tube,  one  inch  square 
C  D  EF  are  the  bellows,  consisting  of  two  boards,  eight  inches 
square,  connected  by  broad  pieces  of  leather,  or  india-rubber 
ploth  in  the  manner  of  a  pair  of  common  bellows.  One  pound 


*  A  paradox  is  something  which  is  seemingly  absurd,  but  true  in  fact.  But 
in  what  is  called  the  Hydrostatic  Paradox  there  is  in  reality  no  paradox  at 
all.  It  is  true  that  a  small  quantity  of  fluid  will  balance  any  quantity, 
however  large,  but  it  is  on  the  same  principle  as  that  with  which  the  longer 
arm  of  the  lever  acts.  In  order  to  raise  the  larger  quantity  of  fluid,  the 
smaller  quantity  must  be  elevated  to  a  height  in  proportion  as  the  bulk  of 
the  larger  quantity  exceeds  the  smaller.  Thus,  to  raise  500  Ibs  of  watei 
by  the  descending  force  of  one  pound,  the  latter  must  descend  500  inohes 
while  the  former  is  rising  one  inch  ;  and  hence,  what  is  called  the  hydro- 
rtatic  paradox  is  in  strict  conformity  with  the  fundamental  principle  of  Me 
p'lauics,  that  what  is  gained  in  power  is  lost  iu  time,  or  hi  space 


HYDROSTATICS. 


119 


of  wator  pour  ,<1  iiito  the  tube  will  raise  sixty-  Pig.  66 

four  pounds   on   the   bellows.     If  a   smaller. 

tube  be  used,  the  same  quantity  of  water  will 

fill  it  higher,  and,  consequently,  will  raise  a 

greater  weight  ;  but,  if  a  larger  tube  be  used, 

it  will,  of  course,  not  fill  it  so  high,  and,  con- 

sequently, will  not  raise  so  great  a  weight, 

because  it  is  the  height,  not  the  quantity,  which 

causes  the  pressure. 

The  hydrostatic  bellows  may  be  constructed 
in  a  variety  of  forms,  the  simplest  of  which 
consists,  as  in  the  figure,  of  two  boards  connected  together  bj 
broad  pieces  of  leather,  or  india-rubber  cloth,  in  such  a  manner 
as  to  allow  the  upper  board  to  rise  and  fall  like  the  common 
bellows.  A  perpendicular  tube  is  so  adjusted  to  this  apparatus 
that  water  poured  into  the  tube,  passing  between  the  boards, 
will  separate  them  by  its  upward  pressure,  even  although  the 
upper  board  is  loaded  with  a  considerable  weight. 

[N.  B.  A  small  quantity  of  water  may  be  poured  into  the  "bellows  to  separate 
the  surfaces  before  they  are  loaded  with  the  weight.] 

How  is  the  force         446.   The  force  of  pressure  exerted  on 


lettoics  estimated  f  tube  is  estimated  by  the  comparative  size 
of  the  tube  and  the  bellows.  Thus,  if  the  tube  be  one  inch 
square,  and  the  top  of  the  bellows  twelve  inches,  thus  con- 
taining 144  square  inches,  a  pound  of  water  poured  into 
the  tube  will  exert  a  pressure  of  144  pounds  on  the  bellows. 
Now  it  will  be  clearly  perceived  that  this  pressure  is  caused 
~by  the  height  of  the  column  of  water  in  the  tube.  A  pound, 
or  a  pint,  of  water  will  fill  the  tube  144  times  as  high  as  the 
same  quantity  would  fill  the  bellows.  To  raise  a  weight  of 
144  pounds  on  the  bellows  to  the  height  of  one  inch,  it  will 
be  necessary  to  pour  into  the  tube  as  much  'water  as  would 
What  f  undo-  fill  the  tube  were  it  144  inches  long.  It  will 
mental  law  of  tlms  be  perceived  that  the  fundamental  prin- 


120 


NATURAL    PHILOSOPHY. 


Fig.  67. 


Mechanics  ciple  of  the  laws  of  motion  is  here  also  in  full 
'hwirostatfc  *°  force — namely,  that  what  is  gained  in  power 
pressure  ?  is  lost  either  in  time  or  in  space  j  for  while 
the  water  in  the  bellows  is  rising  to  the  height  of  one  inch, 
that  in  the  tube  passes  over  144  inches. 
Explain  447.  Another  form  of  apparatus,  by  means  of 
Fig.  67.  which  it  can  be  proved  that  fluids  press  in  proportion 
to  their  perpendicular  height,  and  not  their  quantity,  is  seen  in 
Fig.  67.  This  apparatus  unites  simplicity  with  convenience. 
Instead  of  two  boards,  connected  with  leather,  an  india-rubber 
bag  is  placed  between  two  boards,  connected  by  crossed  bars 
with  a  board  below,  loaded  with  weights,  and  the  upper  boards 
are  made  to  rise  or  fall  as  the  water  runs  into  or  out  of  the 
bag.  It  is  an  apparatus  easily  repaired,  and  the  bag  may  also 
be  used  for  gas,  or  for  experiments  in  Pneumatics 

A  and  B  are  two  vessels  of  unequal  size,  but  of  the  same 
length.  These  may  suc- 
cessively be  screwed  to 
the  apparatus,  and  filled 
with  water.  Weights 
may  then  be  added  to 
the  suspended  scale  until 
the  pressure  is  counter- 
balanced. It  will  then 
be  perceived  that,  al- 
though A  is  ten  times 
larger  than  B,  the  water 
will  stand  at  the  same 
height  in  both,  because 
they  are  of  the  same 
length.  If  C  be  used 
instead  of  A  or  B,  the 
apparatus  may  be  used  as  the  hydrostatic  bellows. 

If  a  cask  be  filled  with  water  and  a  long  pipe  be  fitted  to 
it,  water  poured  into  the  pipe  will  exert  so  great  hydro 
static  pressure  as  to  burst  the  cork. 


HYDROSTATICS. 


/// 

ner 


man- 

hy- 


ployed  as  a 


4i8.  HYDROSTATIC  PRESSURE  USEI>  AS  A 
MECHANICAL  POWER.  —  If  water  be  confined 
*u  an^  vesse^  an(*  a  pressure  to  any  amount 
be  exerted  on  a  square  inch  of  that  water,  a 
pressure  to  an  equal  amount  will  be  trans- 
mitted to  every  square  inch  of  the  surface  of 
the  vessel  in  which  the  water  is  confined. 

449.  This  property  of  fluids  seems  to  invest  us  with  a  power  of 
increasing  the  intensity  of  a  pressure  exerted  by  a  comparatively 
small  force,  without  any  other  limit  than  that  of  the  strength  of 
the  materials  of  which  the  engine  itself  is  constructed.  It  also 
enables  us  with  great  -facility  to  transmit  the  motion  and  force  of 
one  machine  to  another,  in  cases  where  local  circumstances  pre- 
clude the  possibility  of  instituting  any  ordinary  mechanical  con- 
nexion between  the  two  machines.  Thus,  merely  by  means  of 
water-pipes,  very  great  pressures  may  be  transmitted  to  any  dis- 
tance, and  over  inequalities  of  ground,  or  through  any  other  ob- 
structions. (See  par.  1423.) 


On  what  prin-       ^®    ^  *s  on  ^e  Prin°iple  OI>  hydrostatic  press- 
cipJe  is  Bra-      ure  that  Bramah's  hydrostatic  press,  represented 

in  Fi~   gg  jg  constructed.     The  main  features  of 

.  , 

this  apparatus  are  as  iollows  :  a  is  a  narrow,  and 


mah's  hydro- 
static  press 
constructed? 


Explain  Fig.  A  a  large  metallic  cylinder,  having  communi- 
cation one  with  the  other.  Water  stands  in  both 
the  cylinders.  The 
piston  S  carries  a 
strong  head  P,  which 
works  in  a  frame  op- 
posite to  a  similar 
plate  R.  Between 
the  two  plates  trie 
substance  W  to  be 
compressed  is  placed. 
In  the  narrow  tube, 
z  is  a  piston  p, 
worked  by  a  lever 
cf)d,  its  short  arm 


122  NATURAL   H1IL030PHY. 

•j&  driving  the  piston,  while  the  power  is  applied  at  d.  The 
pressure  exerted  by  the  small  piston  p  on  the  water  at  a  is 
transmitted  with  equal  force  throughout  the  entire  mass  of  the 
fluid,  while  the  surface  at  A  presses  up  the  piston  S  with  a 
force  proportioned  to  its  area.  For  instance,  if  the  cylinder  «, 
of  the  force-pump  has  an  area  of  half  an  inch,  while  the  greal 
cylinder  has  an  area  of  200  inches,  then  the  pressure  of  the 
water  in  the  latter  on  the  piston  S  will  be  equal  to  400  times 
that  on  p 

Next,  suppose  the  arms  of  the  lever  to  be  to  each  other  as 
1  to  50,  and  that  at  d,  the  extremity  of  the  longer  arm,  a  man 
works  with  a  force  of  50  pounds,  the  piston  p  will  consequently 
descend  on  the  water  with  a  force  of  2500  pounds.  Deducting 
one-fourth  for  the  loss  of  power  caused  by  the  different  impedi- 
ments to  motion,  and  one  man  would  still  be  able  to  exert  a 
force  of  three-quarters  of  a  million  of  pounds  by  means  of  this 
machine.  This  press  is  used  in  pressing  paper,  cloth,  hay,  gun- 
powder, &c. ;  also  in  uprooting  trees,  testing  ths  strength  of 
ropes,  &c.  (See  pars.  1425,  1426.) 

When  will  one 

fluid  float  on          451.  A  fluid  specifically  lighter  than  another 

the  surface  of    fluid  w{\\  float  Up0n  its  surface> 

another  fluid? 

[N.  B.  This  is  but  another  way  of  stating  the  law  mentioned  in  Nos  409 
and  410.] 

452.  If  an  open  bottle,  filled  with  any  fluid  specifically  lighter 
than  water,  be'  sunk  in  water,  the  lighter  fluid  will  rise  from  the 
oottle,  and  its  place  will  be  supplied  with  the  heavier  water. 

j™        .,,  453.  Any  substance  whose  specific  gravity  is 

body  rise,  sink  greater  than  any  fluid  will  sink  to  the  bottom  of 
or  float,  in  a      that  fluid,  and  a  body  of  the  same  specific  gravity 
with  a  fluid  will  neither  rise  nor  fall  in  the  fluid 
but  will  remain  in  whatever  portion  of  the  fluid  it  is  placed 

*  The  slaves  in  the  West  Indies,  it  is  said,  steal  rum  by  inserting  th« 
long  neck  of  a  bottle,  full  of  water,  through  the  top  aperture  of  the  rum 
aask.  The  water  falls  out  of  the  bottle  i  uto  the  cask,  while  the  light* 

rum  ascends  in  ite  stead 


HYDROSTATICS.  123 

But  a  body  whose  specific  gravity  is  less  than  that  of  a 
fluid  will  float. 

This  is  the  reason  why  some  bodies  will  sink  and  others 
float,  and  still  others  neither  sink  nor  float.*  (See  par.  1427.) 

How  deep  will  454-  A  body  specifically  lighter  than  a  fluid 
a  body  sink  in  will  sink  in  the  fluid  until  it  has  displaced  a  por- 
a  fluid?  tion  of  the  fluid  eual  in  weiht  to 


455.  If  a  piece  of  cork  is  placed  in  a  vessel  of  water,  about  one- 
third  part  of  the  cork  will  sink  below,  and  the  remainder  will  stand 
above,  the  surface  of  the  water  ;  thereby  displacing  a  portion  of 
water  equal  in  bulk  to  about  a  third  part  of  the  cork,  and  this 
quantity  of  water  is  equal  in  weight  to  'the  whole  of  the  cork 
because  the  specific  gravity  of  water  is  about  three  times  as  great 
as  that  of  cork. 

456.  It  is  on  the  same  principle  that  boats,  ships,  &c.,  although 
composed  of  materials  heavier  than  water,  are  made  to  float.     From 
their  peculiar  shape,  they  are  made  to  rest  lightly  on  the  water. 
The  extent  of  the  surface  presented  to  the  water  counterbalances 
the  weight  of  the  materials,  and  the  vessel  sinks  to  such  a  depth  as 
will  cause  it  to  displace  a  portion  of  water  equal  in  weight  to  the 
whole  weight  of  the  vessel.     From  a  knowledge  of  the   specific 
gravity  of  water,  and  the  materials  of  which  a  vessel  is  composed, 
rules  have  been  formed  by  which  to  estimate  the  tonnage  of  vessels  ; 
that  is  to  say,  the  weight  which  the  vessel  will  sustain  without 
sinking. 

standard  for  ^"'  ^e  standard  which  has  been  adopted  to 

estimating  the  estimate  the  specific  gravity  of  bodies  is  rain  or 

specific  grav-  distilled  water  at  tho  temperature  of  60°.t 
ity  of  bodies  ? 

*  The  bodies  of  birds  that  frequent  the  water,  or  that  live  in  the  water, 
are  generally  much  lighter  than  the  fluid  in  which  they  move.  The 
feathers  and  down  of  water-fowl  contribute  much  to  their  buoyancy,  but 
fishes  have  the  power  of  dilating  and  contracting  their  bodies  by  means  of 
an  internal  air-vessel,  which  they  can  contract  or  expand  at  pleasure. 

The  reason  that  the  bodies  of  persons  who  have  been  drowned  first  sink, 
and,  after  a  number  of  days,  will  float,  is,  that  when  first  drowned  the  air 
being  expelled  from  the  lungs,  makes  the  body  specifically  heavier  than 
water,  and  it  will  of  course  sink  ;  but,  after  decomposition  has  taken  place. 
the  gases  generated  within  the  body  distend  it,  and  render  it  lighter  thaa 
water,  and  they  will  cause  it  to  rise  to  the  surface. 

t  As  heat  expands  and  cold  condenses  all  metals,  their  specific  gravity 
cannot  be  the  same  in  summer  that  it  is  in  winter.  For  this  reason,  the} 
will  not  serve  as  a  standard  to  estimate  the  specific  gravity  of  other  bodies 
The  reason  that  distilled  water  is  used  is,  that  spring,  wel',  or  river  water  u 
•eldom  perfectly  pure,  aud  the  various  substances  -mixed  with  it  affect  itf 


124  NATURAL    PHILOSOPHY 

This  is  found  to  be  a  very  convenient  standard,  because  a 
cubic  foot  of  water  at  that  temperature  weighs  exactly  one 
thousand  ounces 

458.  Taking  a  certain  quantity  of  rain  or  distilled  water,  we  find 
that  a  quantity  of  gold,  equal  in  bulk,  will  weigh  nearly  twenty 
times  as  much  as  the  water ;  of  lead,  nearly  twelve  times  as  much  ; 
while  oil,  spirit,  cork,  &c.,  will  weigh  less  than  water.* 

weight.  The  cause  of  the  ascent  of  steam  or  vapor  may  be  found  in  its 
specific  gravity.  It  may  here  be  stated  that  rain,  snow  and  hail,  are  formed 
by  the  condensation  of  the  particles  of  vapor  in  the  upper  regions  of  the 
atmosphere.  Fine,  watery  particles,  coming  within  the  sphere  of  each 
aiher's  attraction,  unite  in  the  form  of  a  drop,  which,  being  heavier  than 
the  air,  falls  to  the  earth.  Snow  and  hail  differ  from  rain  only  in  the 
different  degrees  of  temperature  at  which  the  particles  unite.  When  rain, 
snow,  or  hail  falls,  part  of  it  reascends  in  the  form  of  vapor  and  forma 
clouds,  part  is  absorbed  by  the  roots  of  vegetables,  and  part  descends  into 
the  earth  and  forms  springs.  The  springs  form  brooks,  rivulets,  rivers, 
<fco.,  and  descend  to  the  ocean,  where,  being  again  heated  by  the  sun,  the 
water,  rising  in  the  form  of  papor,  again  forms  clouds,  and  again  descends 
in  rain,  snow,  hail,  <fec.  The  specific  gravity  of  the  watery  particles  which 
constitute  vapor  is  less  than  that  of  the  air  near  the  surface  of  che  earth  ; 
they  will,  therefore,  ascend  until  they  reach  a  portion  of  the  atmosphere  of 
the  same  specific  gravity  with  themselves.  But  the  constant  accession  of 
fresh  vapor  from  the  earth,  and  the  loss  of  heat,  cause  several  particles  to 
come  within  the  sphere  of  each  other's  attraction,  as  has  been  stated  above, 
and  they  unite  in  the  form  of  a  drop,  the  specific  gravity  of  which  being 
greater  than  that  of  the  atmosphere,  it  will  fall  in  the  form  of  rain.  Water, 
as  it  descends  in  rain,  snow  or  hail,  is  perfectly  pure  ;  but,  when  it  has 
fallen  to  the  earth,  it  mixes  with  the  various  substances  through  which  it 
s,  which  gives  it  a  species  of  flavor,  without  affecting  its  transparency. 

*    TABLE   OF   SPECIFIC   GRAVITIES. 
Temperature  about  40J  Fahrenheit. 


Distilled  Water, 

Mercury,  13.596 

Sulphuric  Acid,  1.841 

Nitric  Acid,  1.220 

Prussic  Acid,  .696 

Alcohol  (pure),  .792 

Ether,  .715 

Spirits  of  Turpentine  .869 

Essence  of  Cinnamon,  1.010 

Sea  Water,  1.026 

Milk,  1.030 

Wine,  .993 

Olive  Oil,  .915 

Naphtha,  .847 

Iodine,  4.946 

PUtinum,  22.050 

Goli,  19.360 

Silver,  10.500 

Rhodium,                     «  11.000 


Palladium,  11.500 

Iridiuin,  21,500 

Copper,  8.850 

Lead,  11.250 

Bismuth,  9.8'22 

Tellurium,  6.240 

Antimony,  6.720 

Chromium,  5.900 

Tungsten;  17.500 

Nickel,  8.270 

Cobalt,  7.810 

Tin,  7.293 

Cadmium,  8687 

Zinc,  7.190 

Steel,  7.820 

Iron,  7.788 

Cast-iron,  7.200 

Manga  n«s«,  8.012 

Sodium,  975 


il  IfDKUSTATIUS. 


Hou  is  tk.  459.  The  specific  gravity  of  bodies  that  will 

specific  gravity  •  ^  j     water  is  "ascertained  by  weighing  them 

of  a  body  as-  J 

certamed  when  first  in  water,  and  then  out  of  the  water,  and 

it  is  greater  dividing  the  weight  out  of  the  water  by  tb«  loss 

than  that  of  n       .  ,  ,  . 

water  ?  °^  weight  in  water.     (See  par. 


Potassium, 

Diamond, 

Arsenic, 

Graphite, 

Phosphorus 

Sulphur, 

Lime, 

Galena, 

Marble, 

White  Lead, 

Plaster  of  Paris, 

Nitrate  of  Potash, 

Emerald, 

Garnet, 

Feldspar, 

Serpentine. 

Alum, 

Topaz, 

Bituminous  Coal, 

Anthracite, 

Pulverized  Charcoal, 

Woody  Fibre, 

Lignum  Vitae, 

Boxwood, 


Ash, 


.875 
3.530 
5.670 
2.500 
1.770 
2.08G 
3.150 
7.580 
2.850 
6.730 
2.330 
1.930 
2.700 
3.350 
2.500 
2.470 
1.700 
3.500 
1.250 
1.800 
1.500 
1.500 
1.350 
1.320 
.852 
.845 


Elm, 

Yew, 

Apple  Tree, 

Yellow  Fir,    . 

Cedar, 

Sassafras, 

Poplar, 

Cork  Tree, 

Flint  Glass, 

Pearls, 

Coral, 

China-ware, 

Porcelain  Clay, 

Flint, 

Granite, 

Slate, 

Alabaster, 

Brass, 

Ice, 

Common  Air, 

Hydrogen  Gas, 

Living  Men, 

Brandy, 

Mahogany, 

Chalk, 

Carbonic  Acid  Gas, 


.800 

.807 

.733 

.657 

.561 

.482 

.3»3 

.240 

3.330 

2.750 

2.680 

2.380 

2.2.10 

2.600 

2.700 

2.825 

2.700 

8.300 

.865 

.001 

.000105 

.891 

.820 

1.00? 

1.793 

.001527 


By  means  of  this  table  the  weight  of  any  mass  of  matter  can  be  ascer 
tained,  if  we  know  its  cubical  contents.  A  cubic  foot  of  water  weighs 
exactly  1000  ounces.  If  we  multiply  this  by  the  number  annexed  tc  *ny 
substance  in  this  table,  the  product  will  be  the  weight  of  a  cubic  foot  of 
that  substance.  Thus  anthracite  coal  has  a  specific  gravity  of  1.800.  A 
thousand  ounces,  multiplied  by  this  sum,  produces  1800  ounces,  which  is 
the  weight  of  a  cubic  foot  of  anthracite  coal. 

The  bulk  of  any  given  weight  of  a  substance  may  also  readily  be  ascer- 
tained by  dividing  that  weight  in  ounces  by  the  number  of  ounces  there  ar» 
in  a  cubic  foot.  The  result  will  be  the  number  of  cubic  feet.  The  cube 
root  of  the  number  of  cubic  feet  will  give  the  length,  depth  and  breadth,  of 
the  inside  ol  a  square  box  that  will  contain  it. 

It  is  to  be  understood  that  all  substances  whose  specific  gravity  is  greater 
than  water  will  sink  when  immersed  in  it,  and  that  all  whose  specific 
gravity  is  less  than  that  of  water  will  float  in  it.  Let  us,  then,  tako  a 
quantity  of  water  which  will  weigh  exactly  one  pound  ;  a  quantity  of  the 
substances  specified  in  the  table,  of  the  same  bulk,  will  weigh  as  follows  : 


Platinum, 
Fine  Gold, 
Mercury, 

Lead, 


22.050 
19.360 
18.596 
11.250 


Silver, 
Copper, 
Iron, 
Glass, 


10.500  ioa 
8.850  " 
7,788  « 
3.880  •• 


126 


NATURAL    PHILOSOPHY. 


Describe  tht. 


460.  Fig.  69  represents  the  scales   for  asce» 


teaks  used  for   taining  the  specific  gravity 


finding  the 
specific  grav- 
ity of  a  body. 


Fig.  69 


of  bodies.      One   scale   is 

shorter  than  the  other,  and 

a  hook  is  attached  to  the 
bottom  of  the  scale,  to  which  substances 
whose  specific  gravity  is  sought  may  be 
attached  and  sunk  in  water. 

461.  Suppose  a  cubic  inch  of  gold  weighs  nineteen  ounces  when 
weighed  out  of  the  water,  and  but  eighteen  ounces  *  when  weighed 


Marble, 

Chalk, 

Coal, 

Mahogany, 

Milk, 

Boxwood, 

Rain  Water, 

Oil, 

Ice, 


2.850  Ibs. 
,793   " 
.250  " 

Brandy, 
Living  Men, 

Ash, 

.003  " 
.030   " 
.320   " 

Beecb, 
Elm, 
Fir, 

.000  « 

Cork, 

.920  " 
.865   " 

Common  Air, 
Hydrogen  Gas, 

.820  fbs. 

.891  " 

.845  " 

.852  " 

.800  " 

.«.  7  " 

.240  " 

.0011  « 

.000105  *« 


A  cubic  foot  of  water  weighs  one  thousand  avoirdupois  ounces.  By  mu^ 
tiplying  the  number  opposite  to  any  substance  in  the  above  table  by  on- 
thousand,  we  obtain  the  weight  of  a  cubic  foot  of  that  substance  in  ounce». 
Thus,  a  cubic  foot  of  platinum  is  23,000  ounces  in  weight. 

In  the  above  table  it  appears  that  the  specific  gravity  of  living  men  is 
about  one-ninth  less  than  that  of  common  water.  So  long,  therefore,  as 
the  lungs  can  be  kept  free  from  water,  a  person,  although  unacquainted 
with  the  art  of  swimming,  will  not  completely  sink,  provided  the  hands  and 
arms  be  kept  under  water. 

The  specific  gravity  of  sea-water  is  greater  than  that  of  the  water  of 
fakes  and  rivers,  on  account  of  the  salt  contained  in  it.  On  this  account, 
the  water  of  lakes  and  rivers  has  less  buoyancy,  and  it  is  more  difficult  to 
swim  in  it. 

*  The  gold  will  weigh  less  in  the  water  than  out  of  it,  on  account  of  the 
upward  pressure  of  the  particles  of  water,  which  in  some  measure  supports 
it,  and,  by  so  doing,  diminishes  its  weight.  Now,  as  the  upward  pressure 
of  these  particles  is  exactly  sufficient  to  balance  the  downward  pressure  of 
a  quantity  of  water  of  exactly  the  same  dimensions  with  the  gold,  it  follows 
that  the  gold  will  lose  exactly  as  much  of  its  weight  in  water  as  a  quantity 
of  water  of  the  same  dimensions  with  the  gold  will  weigh.  And  this  rule 
applies  to  all  bodies,  heavier  than  water,  that  are  immersed  in  it.  They 
will  lost,  as  much  of  their  weight  in  water  as  a  quantity  of  water  of  their  own 
dimensions  weighs.  All  bodies,  therefore,  of  the  same  size,  lose  the  same 
quantity  of  their  weight  in  water.  Hence,  th<t  specific  gravity  of  a  body  is  the 
weight  of  it  compared  with  that  of  water.  As  a  body  loses  a  quantity  of  its 
weight  when  immersed  in  water,  it  follows  that  when  the  body  is  lifted 
from  the  ivater  that  portion  of  its  weight  which  it  had  lost  will  be  restored. 
This  is  the  reason  that  a  bucket  of  water,  drawn  from  a  well,  is  heavier 
when  it  rises  above  the  surface  of  the  water  in  the  well  than  it  is  while  it 
remains  below  the  surface.  For  the  same  reason  our  limbs  feel  heavy  is 
fttviug  a  bith 


HYDROSTATICS.  1^ 

m  water,  the  loss  in  water  is  one  ounce.  The  weight  out  of  water, 
nineteen  ounces,  being  divided  by  one  (the  loss  in  water),  gives 
nineteen.  The  specific  gravity  of  gold,  then,  would  be  nineteen  ; 
or  in  other  words,  gold  is  nineteen  times  heavier  than  water. 

462.  The  specific  gravity  of  a  body  that  will 
specific  gravity  not  s^n^  *n  water  ^  ascertained  by  dividing  its 
of  a  body  weight  by  the  sum  of  its  weight  added  to  the 

loss  of  wei'ht  which  ii;  occasions  in  a 


previously  balanced  in  water.*1     (See  par.  1436.) 

463.  If  a  body  lighter  than  water  weighs  six  ounces,  and,  on  being 
attached  to  a  heavy  body,  balanced  in  water,  is  found  to  occasion  it 
to  lose  twelve  ounces  of  its  weight,  its  specific  gravity  is  determined 
by  dividing  its  weight  (six  ounces)  by  the  sum  of  its  weight  added 
to  the  loss  of  weight  it  occasions  in  the  heavy  body  ;  namely,  6 
added  to  12,  which,  in  other  words,  is  6  divided  by  18,  or  ^, 
which  is  J. 

464.    Questions  for  Solution. 

(1.)  A  body  lighter  than  water  caused  the  loss  of  10  Ibs.  to  a  heavier 
body  immersed  in  water.  In  air  the  same  body  weighed  30  Ibs.  What 
was  its  specific  gravity  1 

Solution.  —  30  Ibs.,  its  weight,  divided  by  (30+10=)  40  (the  sum  of  its 
weight  added  to  the  loss  of  weight  which  it  caused  in  another  body  pre- 
vtously  balanced  in  the  water).  Ans.  .75. 

(2.)  A  body  that  weighed  15  Ibs.  in  air  weighed  but  12  in  water.  What 
^as  its  specific  gravity  1  Ans.  5. 

(3.)  If  a  cubic  foot  of  water  weigh  1000  ounces,  what  is  the  weight  of  an 
e  tual  bulk  of  gold  1  Ans.  1210  Id. 

(4.)  The  weight  of  an  equal  bulk  of  lead  1  Ans.  708  II.  2  oz. 

(5.)  The  weignt  of  an  equal  bulk  of  cork  1  Ans.  15  Ib. 

*  The  method  of  ascertaining  the  specific  gravities  of  bodies  was  dis- 
covered accidentally  by  Archimedes.  He  had  been  employed  by  the  King 
of  Syracuse  to  investigate  the  metals  of  a  golden  crown,  which  he  suspected 
had  been  adulterated  by  the  workmen.  The  philosopher  labored  at  the 
problem  in  vain,  till,  going  one  day  into  the  bath,  he  perceived  that  the 
water  rose  in  the  bath  in  proportion  to  the  bulk  of  his  body.  He  instantly 
perceived  that  any  other  substance  of  equal  size  would  raise  the  water  just 
as  much,  though  one  of  equal  weight  and  less  bulk  could  not  produ'ce  the 
same  effect.  He  then  obtained  two  masses,  one  of  gold  and  one  of  silver, 
each  equal  in  weight  to  the  crown,  and  having  filled  a  vessel  very  accu- 
rately with  water,  he  first  plunged  the  silver  mass  into  it,  and  observed  the 
quantity  of  water  that  flowed  over  ;  he  then  did  the  same  with  the  gold, 
and  found  that  a  less  quantity  had  passed  over  than  before.  Hence  he 
inferred  that,  though  of  ef\ual  weight,  the  bulk  of  the  silver  was  greater 
than  that  of  the  gold,  and  that  the  quantity  of  water  displaced  was,  in  encb 
experiment,  equal  to  the  bulk  of  the  metal.  He  next  made  trial  with  the 
crown,  and  found  that  it  displaced  more  water  than  the  gold,  and  less  than 
the  silver,  which  led  him  to  conclude  that  it  was  neither  pure  gold  no/ 
pure  silver 


NATURAL   PHILOSOPHY. 

(6.)     The  weight  of  an  equal  bulk  of  iron  1  A",e.  486  'ft.  12  o* 

(7.)     What  is  the  weight  of  a  cubic  foot  of  mahogany  1    Ans.  62  Ib.  11  era. 

(8.)     The  weight  of  a  cubic  foot  of  marble  \  Ana.  iTQlb.  2  OK. 

(9.)  Wtat  is  the  weight  of  an  iceberg  6  miles  long,  .  mile  wide,  *n<i 
400  feet  thick  1  A  ns.  904,304.600  tons. 

(10.)  What  is  the  weight  of  a  marble  statue,  supposing  it  to  be  exactly 
a  yard  and  half  of  cubic  measure  1  Ans.  7214,06  #>.  -f- 

(11.)  If  a  cubical  body  of  cork*exactly  9  inches  on  each  side  be  placed 
in  water,  how  deep  will  it  sink  1  Ans.  2.16  in. 

(12.)  Suppose  that  4  boats  were  made  each  out  of  one  of  the  following 
kinds  of  wood,  namely,  ash,  beech,  elm  and  fir,  which  would  carry  tfhe 
greatest  weight  without  sinking  1  Ans.  That  ojfir. 

What  is  an  465.  An  Hydrometer  is  an  instrument  to  ascer- 

Hydrometer?     tain  the  specific  gravity  of  liquids.      (See  par. 
and  on  what        -[Aof\\ 
principle  is  it 

constructed?  466>  T^  hydrometer  is  constructed  on  the 
principle  that  the  greater  the  weight  of  a  liquid,  the  greater  will 
be  its  buoyancy. 

How  is  an  hy-  ^67.  The  hydrometer  is  made  in  a  variety  of 
drometer  con-  foras,  but  it  generally  consists  of  a  hollow  ball 
of  silver,  glass,  or  other  material,  with  a  gradu- 
ated scale  rising  from  the  upper  part.  A  weight  is  attached 
Otslow  the  ball.  When  the  instrument  thus  constructed  is  im- 
mersed in  a  fluid,  the  specific  gravity  of  the  fluid  is  estimated  by 
the  portion  of  the  scale  that  remains  above  the  surface  of  the 
fluid.  The  greater  the  specific  gravity  of  the  fluid,  the  less  will 
the  scale  sink. 

Of  what  use  ^68.  The  hydrometer  is  a  very  useful  instru- 
1*5  the  hydrom-  ment  for  ascertaining  the  purity  of  many  articles 
in  common  use.  It  sinks  to  a  certain  determinate 
depth  in  various  fluids,  and  if  the  fluids  be  adulterated  the  hy- 
drometer will  expose  the  cheat.  Thus,  for  instance,  the  specific 
gravity  of  sperm  oil  is  less  than  that  of  whale  oil,  and  of  course 
has  less  buoyancy.  If.  therefore  the  hydrometer  does  not  sink 
to  the  proper  mark  of  sperm  oil,  it  will  at  once  be  seen  that  the 
Article  is  not  pure. 

Of  what  does  Hy-  469.  HTDEAULICS. — Hydraulics  treats  of 
draulics  treat?  liquids  in  motion,  and  the  instruments  by 
whicli  their  motion  is  guided  01  controlled.  (See  par.  1437.) 


HYL>KAUL1CS.  1L5J* 

470  This  branch  of  Hydrodynamics  describes  tLj  ejects  ot 
liquids  issuing  from  pipes  and  tubes,  orifices  or  apertuies,  the 
motion  of  rivers  and  canals,  and  the  forces  developed  in  the 
action  of  fluids  with  solids. 

471.  The  quantity  of  a  liquid  discharged  in  a 
&iven  time  tlm>ugh  a  P^P6  or  orifice  is  equal  to  a 
be  discharged  column  of  the  liquid  having  for  its  base  the  orifice 
from  an  orifice  or  the  area  of  the  bore  of  the  pip6j  and  a  helght 

given  size  ?        equal  to  the  space  through  which  the  liquid  would 
pass  in  the  given  time. 

472.  Hence,  when  a  fluid  issues  from  an  orifice  in  a  vessel,  it  ia 
discharged  with  the  greatest  rapidity  when  the  vessel  from  which  it 
flows  is  kept  constantly  full.*  This  is  a  necessary  consequence  of 
the  law  that  pressure  is  proportioned  to  the  height  of  the  column 
above. 

From  what  orifice  ^73.  When  a  fluid  spouts  from  several  orifices 
will  a  fluid  snout  *n  *ne  8^e  °^  a  vessel>  it  is  thrown  with  the 
to  the  greatest  greatest  random  from  the  orifice  nearest  to  the 

distance  ?  centre — the  random  being   measured  horizon- 

tally from  the  bottom  of  the  vessel. 

474.  A  vessel  filled  with  any  liquid  will  discharge  a  greater  quan- 
tity of  the  liquid  through  an  orifice  to  which  a  short  pipe  of  pecu- 
liar shape  is  fitted,  than  through  an  orifice  of  the  same  size  without 
a  pipe.  _  (See  par.  1457.) 

This  is  caused  by  the  cross-currents  made  by  the  rushing  of  the 
water  from  different  directions  towards  the  sharp-edged  orifice. 
The  pipe  smooths  the  passage  of  the  liquid.  But,  if  the  pipe  pro- 
ject into  the  vessel,  the  quantity  discharged  will  be  diminished, 
instead  of  increased,  by  the  pipe. 

475.  The  quantity  of  a  fluid  discharged  through  a  pipe  or  an 
orifice  is  increased  by  heating  the  liquid  ;  because  heat  diminishes 
the  cohesion  of  the  particles,  which  exists,  to  a  certain  degree,  in 
all  liquids. 

476.  Water,  in  its  motion,  is  retarded  by  the 
a  current  of  ^^ion  °f  tne  bottom  and  sides  of  the  channel 
water  flows  through  which  it  passes.  For  this  reason,  the 

most  rapidly,  veiocity  Of  tne  surface  of  a  running  stream  is 
and  why  ?  J 

always  greater  than  that  of  any  other  part. 

*  The  velocity  with  which  a  liquid  issues  from  an  infinitely  small  orifice 
in  ihe  bottom  or  sides  of  a  vessel  that  is  kept  full  is  equal  to  that  which  a 
Heavy  body  would  acquire  by  falling  from  the  level  o?  khe  surface  to  thf 
of  the  orifice.  —  Br«ndt. 


180  NATUKAL   PHILOSOPHY. 

477  In  consequence  of  the  friction  of  the  banks  and  beds  of 
rivers,  and  the  numerous  obstacles  they  meet  in  their  circuitous 
course,  their  progress  is  slow.  If  it  were  not  for  these  impediments, 
the  velocity  which  the  waters  would  acquire  would  produce  very  dis- 
astrous consequences.*  An  inclination  of  three  inches  in  a  mile,  in 
the  bed  of  a  river,  will  give  the  current  a  velocity  of  about  three 
uiiles  an  hour. 

478.  To  measure  the  velocity  of  a  stream  at  its  surface,  hollow 
floating  bodies  are  used  ;  as,  for  example,  a  glass  bottle  filled  with 
a  sufficient  quantity  of  water  to  make  it  sink  just  below  the  level  of 
the  current,  and  having  a  small  flag  projecting  from  the  cork.  A 
wheel  may  also  be  caused  to  revolve  by  the  current  striking  against 
boards  projecting  from  the  circumference  of  the  wheel,  and  the 
rapidity  of  the  current  may  be  estimated  by  the  number  of  the  rev- 
olutions in  a  given  time. 

How  may  the        479.     The  velocity  of  a  current  of  water  at  any 

portion  of  its  depth  may  be 
depth  be  ascer-  ascertained  by  immersing  in 
iained?  ft  a  ^nt  tube,  shaped  like  a 

tunnel  at  the  end  which  is  immersed. 

480.     Fig.  70  is  a  tube  shaped  like  a 
tunnel,  with  the  larger  end  immersed  in  an 
opposite   direction   to   the   current.     The 
rapidity  of  the  current  is  estimated  by  the  _jj 
height  to  which  the  water  is  forced  into  the 
tube,  above  the  surface  of  the  current.    By 
such  an  instrument  the  comparative  velocity 
of  different  streams,  or  the  same  stream  at  different  times,  may 
be  estimated. 

How  are  waves  481.  Waves  are  caused,  first,  by  the  friction 
caused?  between  air  and  water,  and  secondly,  and  on  a 

much  grander  scale,  by  the  attraction  of  the  sun  and  moon 
exerted  on  the  surface  of  the  ocean,  producing  the  phenomena 
of  the  tides. 

482.  The  hand  of  a  wise  and  benevolent  Creator  is  seen  in  nothing 
more  clearly  than  in  the  laws  and  operations  of  the  material  world. 
Were  it  not  for  the  almost  ceaseless  motion  of  the  water,  the  ocea? 

*  See  what  is  stated  with  regard  to  fr  ction  in  Nos.  373  and  374. 


HYDRAULICS. 


What  are  the 
principal  hy- 
draulic instru- 
ments or  ma- 
:hines  ? 


itself  would  become  unbearable.  Decayed  and  decaying  matter 
would  be  constantly  emitting  pestilential  vapors,  poisoning  the  at- 
mosphere, and  spreading  contagion  and  death  far  beyond  the  borders 
of  the  ocean.  The  "  ceaseless  motion  "  distributes  the  poisonous  in- 
gredients, and  aids  tliat  change  which  renders  them  harmless. 

483.  The  equilibrium  of  a  fluid,  according  to  recent  discoveries, 
cannot  be  disturbed  by  waves  to  a  greater  depth  than  about  three 
hundred  and  fifty  times  the  altitude  of  the  wave. 

484.  When  oil  is  poured  on  the  windward  side  of  a  pond,  the 
whole  surface  will  become  smooth.     The  oil  protects  the  water  from 
the  friction  of  the  wind  or  air.     It  is  said  that  boats  have  been  pre- 
served in  a  raging  surf,  iu  consequence  of  the  sailors  having  emptied 
a  barrel  of , oil  on  the  water. 

485.  The   instruments   or   machines    for 
raising   or   drawing   water   are»the   common 
pump,  the  forcing-pump,  the  chain-pump,  the 
siphon,  the  hydraulic  ram,  and  the  screw  of, 
Archimedes. 

[The  common  pump  and  the  forcing-pump  will  be  Fig.  71. 

aoticed  in  connexion  with  Pneumatics,  as  their  opera- 
tion is  dependent  upon  principles  explained  in  that 
department  of  Philosophy.  The  fire-engine  is  nothing 
more  than  a  double  forcing-pump,  and  will  be  noticed  in 
*»ne  same  connexion.] 

486.  The  Chain-pump  is 
a  machine  by  which  the  water 
is   lifted   through  a  box  or 

channel,  by  boards   fitted  to  the  channel 

and  attached  to  a  chain.     It  has  been  used 

principally  on  board  of  ships. 

487.  Fig.  71  represents  a  Chain- 
pump.  It  consists  of  a  square  box 
through  which  a  number  of  square 

ooards   or  buckets,  connected  by  a  chain,  is 

ir^de  to  pass.  The  chain  passes  over  the  wheel 

C  and   under  the  wheel  D,  which   is  under 

crater.     The  buckets  are  made  to  fit  the  box, 

*  The  undulations  of  large  bodies  of  water  have  also  produced  material 
jhanges  on  the  face  of  the  globe,  purposely  designed  by  Creative  T~~ 
working  by  secondary  causes,  the  uses  of  which  are  described  in  the 
of  Oeologj 


Wliat    is  the 
nhain-pump  ? 


132  NATUEAL   PHILOSOPHY. 

so  as  to  move  with  little  friction.  The  upper  wheel  C  is  turned 
by  a  crank  (not  represented  in  the  Fig.),  which  causes  the  chain 
with  the  buckets  attached  to  pass  through  the  box.  Each 
bucket,  aj5  it  enters  the  box,  lifts  up  the^vater  above  it,  and 
discharges  it  at  the  top. 

488.     The  screw  of  Archimedes  is  a  ma- 
What  is   the    chjne  ggj^  £0  have  been  invented  by  the  plr- 
chimldes  ?         losopher  Archimedes,  for  raising   water   and 
draining  the  lands  of  Egypt,  about  two  hun- 
dred years  before  the  Christian  era. 

Fig.  72  repre- 
Ezplain  gents  tne  screw  Of 

Archimedes.  A 
single  tube,  or  two  tubes, 
are  wound  in  the  form  of 
a  screw  around  a  shaft  or 
cylinder,  supported  by  the  gs 
prop  and  the  pivot  A,  and 
turned  by  the  handle  n. 
As  the  end  of  the  tube  dips  into  the  water,  it  is  filled  with  the 
fluid,  which  is  forced  up  the  tube  by  every  successive  revolution, 
until  it  is  discharged  at  the  upper  end. 

What  is  the  489.  The  Siphon  is  a  tube  bent  in  the  form 
Siphon  f  Of  foQ  ie^er  "TJ,  one  side  being  a  little  longer 
than  the  other,  to  contain  a  longer  column  of  the  fluid. 

Explain  490.  Fig.  73  represents  a  Siphon.  A  siphon  Kg.  73. 
Fig.  73.  is  used  by  fining  it  with  water  or  some  other 
fluid,  then  stopping  both  ends,  and  in  this  state  immers- 
ing the  shorter  leg  or  side  into  a  vessel  containing  a 
liquid.  The  ends  being  then  unstopped,  the  liquid  will 
run  through  the  siphon  until  the  vessel  is  emptied.  In 
performing  this  experiment,  the  end,  of  the  siphon  which 
is  out  of  the  water  must  always  be  below  the  surface  of  the 
water  in  the  vessel. 


HYDRAULICS.  133 

On  what  prin-  491  The  principle  on  which  the  siphon  acts 
ciple  does  the  js  tnat  tne  ionger  column  having  the  greater 
siphon  act?  '  & 

hydrostatic  pressure,  the  fluid  will  run  down  in 

the  dhection  of  that  column.  The  upward  pressure  in  the 
smaller  column  will  supply  a  continued  stream  so  long  as  that 
column  rests  below  the  surface  of  the  water. 

[N.  B.  This  principle  will  be  better  understood  after  the  principle  is  ex- 
plained on  which  the  operation  of  the  common  pump  depends  ;  for  the 
upward  and  downward  pressure  both  depend  on  the  pressure  of  the  atmos 
phere.] 

492.  The  siphon  may  be  used  in  exemplifying  the  equilibrium  ol 
fluids  ;  for,  if  the  tube  be  inverted  and  .two  liquids  of  different 
density  poured  into  the  legs,  they  will  stand  at  a  height  in  an  in- 
verse proportion  to  their  specific  gravity.  Thus,  as  the  specific 
gravity  of  mercury  is  thirteen  times  greater  than  that  of  water,  a 
column  of  mercury  in  one  leg  will  balance  a  column  of  water  in  the 
other  thirteen  times  higher  than  itself.  But,  if  but  one  fluid  be 
poured  into  both  legs,  that  fluid  will  stand  at  equal  height  in  both 

Explain  the  toy  ^93.  The  toy  called  Tantalus'  *  Cup  consists 
called  Tantalus'  of  a  goblet  containing  a  wooden  figure,  with  a 
^UP'  siphon  concealed  within.  The  water  being 

poured  into  the  cup  until  it  is  above  the  bend  of  the  siphon, 
rises  in  the  shorter  leg,  which  opens  into  the  cup,  and  runs  out 
at  the  longer  end,  which  pierces  the  bottom. 

Fig.  74. 

494.  Fig.  74  represents  the  cup  with  the  siphon, 
the  figure  of  the  man  being  omitted,  in  order  that  the 
position  of  the  siphon  may  be  seen. 

495.  THE  HYDRAULIC  RAM  +  is  an  i 

What  is  the  Hy-       .  ,.  L    ,   f      ' 

draulic  Ram  ?      nious  machine,  constructed  for  the  purpose 

of  raising  water  by  means  of  its  own  im- 
pulse or  momentum. 

*  Tantalus,  in  Heathen  mythology,  is  represented  as  the  victim  of  per- 
petual thirst,  although  placed  up  to  the  chin  in  a  pool  of  water  ;  for,  as  soon 
as  he  attempts  to  stoop  to  drink,  the  water  flows  away  from  his  grasp  ; 
hence  our  English  word  tantalize  takes  its  origin.  In  the  toy  described 
above,  the  siphon  carries  the  water  away  before  it  reaches  the  mouth  of  the 
figure. 

f  The  Hydraulic  Ram,  sometimes  called  by  its  French  name,  Better  Hy- 


134  NATURAL   PHILOSOPHY. 

496  In  the  construction  of  an  hydraulic  ram,  there  musst  no, 
in  the  first  place,  a  spring  or  reservoir  elevated  at  least  four  01 
five  feet  above  the  horizontal  level  of  the  machine/* 

Secondly,  a  pipe  must  conduct  the  water  from  the  reservoir 
to  the  machine  with  a  descent  at  least  as  great  as  one  inch  for 
every  six  feet  of  its  length. 

Thirdly  a  channel  must  be  provided  by  which  the  superflu- 
ous water  may  run  off. 

497.  The  ram  itself  consists  of  a  pipe  having  two  apertures, 
both  guarded  by  valves  of  sufficient  gravity  to  fall  by  their  own 
weight,  one  of  which  opens  downwards,  the  other  opening  up- 
wards into  an  air-tight  chamber.  An  air-vessel  is  generally 
attached  to  the  chamber,  for  the  purpose  of  causing  a  steady 
stream  to  flow  from  the  chamber,  through  another  pipe,  to  the 
desired  point  where  the  water  is  to  be  discharged. 

Explain  the  con-         498'  Fig'  75  rePresents  the  hydraulic  ram. 

struction  of  the  A  B  represents  the  tube,  or  body  of  the  ram, 
having two  apertures,  C  and  D,  both  guarded  by 
valves ;  C  opening  downwards,  D  opening  up- 

draulique,  in  its  present  form,  was  invented  by  Montgolfier,  of  Montpelier 
An  instrument  or  machine  of  a  similar  construction  had  been  previously 
constructed  by  Mr.  Whitehurst,  at  Chester,  but  much  less  perfect  in  ita 
mode  of  action,  as  it  required  to  be  opened  and  shut  by  the  hand  by 
means  of  a  stop-cock.  Montgolfier's  machine,  on  the  contrary,  is  set  in 
motion  by  the  action  of  the  water  itself. 

*  Such  an  elevation  may  easily  be  obtained  in  any  brook  or  stream  of 
running  water  by  a  dam  at  the  upper  part  of  the  stream,  to  form  a  reser- 
voir. It  has  been  calculated  that  for  every  foot  of  fall  in  the  pipe  running 
from  the  reservoir  to  the  ram  sufficient  power  wjll  be  obtained  to  raise 
about  a  sixth  part  of  the  water  to  the  height  of  ten  feet.  With  a  fall  of  only 
four  feet  and  a  half,  sixty-three  hundred  gallons  of  water  have  been  raised 
to  the  height  of  one  hundred  and  thirty-four  feet.  But,  the  higher  the  res- 
ervoir, the  greater  the  force  with  which  the  hydraulic  ram  will  act.  The  ope 
ration  of  the  principle  by  which  the  hydraulic  ram  acts  is  familiar  to  those 
who  obtain  water  for  domestic  purposes  by  means  of  pipes  from  an  elevated 
reservoir,  as  is  the  case  in  many  of  our  large  cities.  A  sudden  stoppage  of 
the  flow,  by  turning  the  cock  too  quickly,  causes  a  jarring  of  the  pipes,  which 
is  distinctly  perceived,  and  often  loudly  heard  all  over  the  building.  This 
is  due  to  the  sudden  change  from  a  state  of  rapid  motion  to  a  state  of  rest. 
The  ineitia  of  the  fluid,  or  its  resistance  to  a  change  from  a  state  of  rapid  mo- 
tion to  a  state  of  rest,  a  property  which  it  possesses  in  common  with  all  other 
kinds  of  matter,  explains  the  cause  of  the  violent  jarring  of  the  pipes,  the 
stopping  of  which  arrests  the  motion  of  the  fluid  ;  and  the  violence,  which 
is  in  exact  proportion  to  the  momentum  of  the  fluid,  is  sometimes  po  jjreal 
as  to  burst  the  pipes 


HYDRAULICS. 


135 


vrards,  and  both  falling  by  their  own  weight.  .Let  us  now  suppose 
the  valve  C  to  be  open  and  D  shut.  The  water,  descending  through 
the  tube  A.  B  with  a  force  proportionate  to  the  height  of  the 


Fig.  75. 


reservoir,  forces  up  the  valve  C  and  closes  the  aperture,  thus 
suddenly  arresting  the  current,  and  causing,  by  its  reaction,  a 
pressure  throughout  the  whole  length  of  the  pipe  ;  this  pressure 
forces  up  the  valve  D,  and  causes  a  portion  of  the  water  to  enter 
the  chamber  above  D.  The  current  having  thus  spent  its  force, 
the  valve  0  immediately  falls  by  its  own  weight,  by  which 
means  the  current  is  again  permitted  to  flow  towards  the  aper- 
ture C.  The  pressure  at  D  thereby  being  removed,  that  valve 
immediately  falls,  and  closes  the  aperture.  When  this  takes 
place,  everything  is  in  the  same  state  in  which  it  was  at  first. 
The  water  again  begins  to  flow  through  the  aperture  at  C,  again 
closing  that  valve,  and  again  opening  D  ;  and  the  same  effects  are 
repeated  at  intervals  of  time,  which,  for  the  same  ram,  undergo 
but  little  variation. 

The  water  being  thus  forced  into  the  chamber  E,  as  it  cannot 
return  through  the.  valve  D,  it  must  proceed  upwards  through 
the  pipe  G,  aad  is  thus  carried  to  any  desired  point  of  dis- 
charge. An  air-vessel  is  frequently  attached  to  the  chamber 


136 


NATURAL    PHILOSOPHY. 


of  the  ram,  which  performs  the  same  office  as  it  does  in  the 
forcing-pump,  namely,  to  cause  a  steady  stream  to  flow  from 
the  pipe  Gr.  The  action,  both  of  the  ram  and  the  forcing-pump, 
without  the  air-vessel,  would  be  spasmodic.^ 

How  are  Springs  499'  SPRINGS  AND  RIVULETS.—  Springs  and 
and  Rivulets  Rivulets  are  formed  by  the  water  from  rain, 

foi-med?  snow,  &c.,  which   penetrates   the   earth,   and 

descends  until  it  meets  a  substance  which  it  cannot  penetrate, 
A  reservoir  is  then  formed  by  the  union  of  small  streams  under 
ground,  and  the  water  continues  to  accumulate  until  it  finds  an 
outlet. 

Flf.  76. 


Fig.  76  represents  a  vertical  section  of  the  crust  of  the  eart* 
",  c,  and  e  are  strata,  -either  porous,  or  full  of  cracks,  which  per 
niit  the  water  to  flow  through,  while  b,  d  and  /,  are  impervious 
to  the  water.  Now,  according  to  the  laws  of  hydrostatics,  the 
water  at  b  will  descend  and  form  a  natural  spring  at  g :  at  i  it 
will  run  with  considerable  force,  forming  a  natural  jet ;  and  at 
I,  p  and  g,  artesian  wells  may  be  dug,  in  which  the  water  will 
rise  to  the  respective  heights  g  h,  p  k,  and  I  m,  the  water  not 

*  The  simplicity  and  economy  of  this  mode  of  raising  water  have  caused 
it  to  be  quite  extensively  adopted  in  the  Northern  States.  When  well  con- 
structed, an  hydraulic  ram  will  last  for  years,  involving  no  additional 
trouble  and  expense,  more  than  occasionally  leathering  the  valves  when 
they  have  been  too  much  worn  by  friction.  The  origin  of  the  name  will  be 
readily  perceived  from  the  mode  of  its  action. 

*'  Et  potum  pastas  age,  Tityre  et  inter  agendum, 
Ocoursare  capro,  curnuferit  ille,  caveto  " —  Virg.  Bucolic  3,  r.  2i 


faYDft  AH  LICS. 


137 


b»?ing  allowed  to  come  in  contact  with  the  porous  soil  through 
which  the  bore  is  made,  but  being  brought  in  pipes  to  the  sur- 
face ;  at  n  the  water  will  ascend  to  about  o.  and  there  will  be 
no  fountain.     This  explains,  also  the  manner  in  which  water  i 
obtained  by  digging  wells. 

How  high  will  50°-  A  sPrin8  wil1  rise  nearl7  as  hi8h'  but 
the  water  of  a  cannot  rise  higher  than  the  reservoir  from 
spring  rise?  whence  it  issues. 

Friction  prevents  the  water  from  rising  quite  as  high  as  the  reser- 
voir. 

Co  what  height        501.  Water  maybe  conveyed  over  hills  and  val- 

may  water   be  leys  in  bent  pipes  and  tubes,  or  through  natural 

conveyed       in  passages,  to  any  height  which  is  not  greater  than 

tubes  ?  the  level  of  the  reservoir  from  whence  it  flows. 

502.  The  ancient  Romans,  ignorant  of  this  property  of  fluids, 
constructed  vast  aqueducts  across  valleys,  at  great  expense,  to  con- 
vey water  over  them.  The  moderns  effect  the  same  object  by  meana 
of  wooden,  metallic,  or  stone  pipes. 

How  arefoun-  503.  Fountains  are  formed  by  water  carried 
tains  formed?  through  natural  or  artificial  ducts  from  a  reser- 
voir. The  water  will  spout  from  the  ducts  to  nearly  the  height 
of  the  surface  of  the  reservoir.  (See  par.  1456.) 

504.  In  Fig.  76  a  fountain  is  represented  at  i, 
issuing  from  the  reservoir,  the  height  of  which  is 
represented  by  a  c.  The  jet  at  i  will  rise  nearly 
as  high  as  c. 

505.  A  simple  method  of  making  an  artificial 
fountain  may  be  understood  by  Fig.  77.  A 
glass  siphon  a  b  c  is  immersed  in  a  vessel  of 
water,  and  the  air  being  exhausted  from  the 
siphon,  a  jet  will  be  produced  at  <z,  proportioned 
to  the  fineness  of  the  bore  and  the  length  of  the 
tube. 

[N.  B.  The  force  of  this  kind  of  artificial  jet  is  in 
»  great  measure  dependent  on  a  pneumatic  principle.] 


Explain  the 
fountain  by 
Fig.  76. 


»38  NATURAL    PHILOSOPHY. 

506.  HERO'S  FOUNTAIN.  —  The  hydraulic  instrument  called 
Hero's  Fountain  is  an  a'pparatus  for  projecting  water  by  means 
of  the  pressure  of  confined  air. 

Fig.  78  represents  Hero's  Fountain.     It  consists  of  two  ves 
rig.  78  se^s»  b°th  air-tight,  and   communicating  by   a 

pipe,  which,  being  inserted  into  the  top  of  the 
lower  vessel,  reaches  nearly  to  the  top  of  the 
upper  vessel,  which  is  in  two  parts,  the  upper 
part  being  filled  with  water,  which  descends  in 
a  pipe,  seen  on  the  right  in  the  figure  to  the 
lower  vessel,  and,  as  it  fills  the  lower  vessel 
condenses  the  air,  forcing  it  up  through  the  left- 
hand  pipe,  and  causing  it  to  press  on  the  sur- 
face of  the  water  in  the  lower  part  of  the  upper 
vessel.  The  water  in  the  upper  vessel  is  thus 
forced  through  the  central  pipe  in  a  jet,  to'  a 
height  nearly  as  great  as  the  length  of  the  pipe  on  the  right. 
The  supply  of  water  is  furnished  in  the  upper  part  of  the  upper 
vessel,  which  may  always  be  kept  full  by  any  external  supply. 
Haw  does  507.  MECHANICAL  AGENCY  OF  FLUIDS. 

a'mecMM      -Water  becomes   a  mechanical    agent    of 
agent?  great  power  by  means    of   its   weight,  its 

momentum  and  its  fluidity.     (See  par.  1450.) 

It  is  used  as  the  moving  power  of  presses,  to  raise  portions  of 
itself,  and  to  propel  or  turn  wheels  of  different  constructions,  which, 
being  connected  with  machinery  of  various  kinds,  form  mills  and 
other  engines  capable  of  exerting  great  force. 


What  is  £^8.  PNEUMATICS.  —  Pneumatics  treats  of 

Pneumatics?  the  mechanical  properties  and  effects  of  air 
and  similar  fluids,  called  elastic  fluids  and  gases,  or  aeri- 
form fluids.  (See  par.  1460.) 

What  is  meant  509.  Aeriform  fluids  are  those  which  have  the 
Tyu  an  aeriform  „  „  .  ,..  „  ,,  .  .,  .  , 

fluid?  form  of  air.     Many  of  them  are  invisible,*  or 

*  Gases  are  all  invisible,  except  when  colored,  which  happens  only  in  a 
few  instances. 


PNEUMATICS.  oD 

nearly  so,  and  all  of  them  perform  very  important  operations  in 
the  mateiial  world.  But,  notwithstanding  that  they  are  in 
most  instances  imperceptible  to  our  sight,  they  are  really 
material,  and  possess  all  the  essential  properties  of  matter. 
They  possess,  also,  in  an  eminent  degree,  all  the  properties 
which  have  been  ascribed  to  liquids  in  general,  besides  others 
by  which  th  3y  are  distinguished  from  liquids. 

,-rr,   .  .   J7  510.  Elastic  fluids  are  divided  into  two  classes. 

What  is  the 

difference  be-      namely,  permanent  gases  and  vapors.     The  gases 

tween  a  perma-  cannot  be  converted  into  the  liquid  state  by  any 
nent  gas  and  ,  „  „.  .  .,  ,., 

a  vapor  ?  known  process  ot  art  ;*but  the  vapors  are  readily 

reduced  to  the  liquid  form  either  by  pressure  or 
diminution  of  temperature.  There  is,  however,  no  essential  dif- 
ference between  the  mechanical  properties  of  both  classes  of  fluids. 


Wh  ub'  t  **^"  ^"s  ^e  a*r  w^cn  we  breathe,  and.  which 
are  embraced  surrounds  us,  is  the  most  familiar  of  all  this  class 
in  the  science  Of  bodies,  it  is  generally  selected  as  the  subject 
of  Pneumatics.  But  it  must  be  premised  that 
the  same  laws,  properties  and  effects,  which  belong  to  air,  belong 
in  common,  also,  to  all  aeriform  fluids  or  gaseous  bodies. 

512.  There  are  two  principal  properties  of  air," 
What  are  the  namely,  gravity  and  elasticity.  These  are  called 
wo  principal  th^  principal  properties  of  this  class  of  bodies. 
Because  they  are  the  means  by  which  their  pres- 


gaseous  bodies?  ence  and  mechanical  agency  are  especially  ex- 

hibited. 
What  degree        513.   Although  the  aeriform  fluids  all  have 

oj  cohesive  at-    ^eight,  they  appear  to  possess  no  cohesive  at- 
J     rr 


traction  home 

traction. 


514.  The  great  degree  of  elasticity  possessed  by  all  aeriform 
fluids,  renders  them  susceptible  of  compression  and  expansion  to  ail 
almos^  unlimited  extent.  The  repulsion  of  their  particles  canse« 
them  to  expand,  while  within  certain  limits  they  are  easi'y  com- 

*  Carbonic  acid  gas  forms  an  exception  to  th4s  remark.  Water  also  is 
the  union  of  oxygen  and  hydrogen  gas. 

6* 


140  NATURAL   PHILOSOPHY. 

pressed.  This  materially  affects  the  state  of  density  and  rarity 
under  which  they  are  at  times  exhibited.* 

,,„      ,  515.  It  may  here  be  stated,  that  all  the  laws 

pertain  to  aeri-  and  properties  of  liquids  (which  have  been  de~ 

form  bodies  in  scribed    under   the    heads    of  Hydrostatics   and 

£en  Hydraulics)  belong  also  to  aeriform  fluids. 

The  chemical  properties  of  both  liquids  and  fluids  belong  pecu- 
fiarly  to  the  science  of  Chemistry,  and  are  not,  therefore,  considered 
in  this  volume. 

What  is  the  ^®'  ^e  air  which  we  breathe  is  an  elastic 

air  which  we      fluid,  surrounding  the  earth,  and  extending 
forty  or  fifty  miles  above  its  surface,  and  con- 
stantly decreasing  upwards  in  density. 

.  517.  It  has  already  been  stated  that  the  air 

air  in  its  most  near  tne  surface  of  the  earth  bears  the  weight  of 
condensed  that  which  is  above  it.  Being  comprespod,  there- 
/orm,  and  for^  ^  ^  we^nt  of  tnat  aboye  jt}  it  mugt  exigt 

in  a  condensed  form  near  the  surface  of  the 
earth,  while  in  the  upper  regions  of  the  atmosphere,  where 
there  is  no  pressure,  it  is  highly  rarefied.  This  condensation, 
or  pressure,  is  very  similar  to  that  of  water  at  great  depths'  m 
the  sea.f 

518.  As  the  air  diminishes  in  density  upwards,  it  follows 
that  it  must  be  more  rare  upon  a  hill  than  on  a  plain.  In  very 
elevated  situations  it  is  so  rare  that  it  is  scarcely  fit  for  respir- 
ation or  breathing,  and  the  expansion  which  takes  place  in  the 
more  dense  air  contained  within  the  body  is  often  painful.  It 


*  The  terms  '*  rarefaction  "  and  "  condensation,"  and  "  rarefied  "  and  "  con 
densed,"  must  be  clearly  understood  in  this  connexion.  They  are  applied 
respectively  to  tho  expansion  and  compression  of  a  body. 

f  The  air  is  necessary  to  animal  and  vegetable  life,  and  to  combustion. 
It  is  a  very  heterogeneous  mixture,  being  filled  with  vapors  of  all  kinds. 
It  consists,  however,  of  two  principal  ingredients,  cal'e»l  oxygen  ani 


occasions  distension,  and  sometimes  causes  the  bursting,  of  the 
smaller  blood-vessels  in  the  nose  and  ears.  Besides,  in  such 
situations  we  are  more  exposed  both  to  heat  and  cold  ;  for, 
though  the  atmosphere  is  itself  transparent,  its  lower  regions 
abound  with  vapors  and  exhalations  from  the  earth,  which  float 
in  it,  and  act  in  some  degree  as  a  covering,  which  preserves  us 
equally  from  the  intensity  of  the  sun's  rays  and  from  the 
severity  of  the  cold. 

519.  Besides  the  two  principal  properties,  gravity  *  and  elasticity, 
the  operations  of  which  produce  most  of  the  phenomena  of  Pneu- 
matics, it  will  be  recollected  that  as  air,.  although  an  invisible  is 
yet  a  material  substance,  possessing  all  the  common  properties  of 
matter,  it  possesses  also  the  common  property  of  impenetrability 
This  will  be  illustrated  by  experiments. 


? 


Where  is  the          **^.  -^e  pressure  of  the  atmosphere  caused 
pressure  of  the  by  its  weight  is  exerted  on  all  substances,  inter- 

nally  and  externally,  and  it  is  a  necessary  conse- 
J  J'  J 


r 
What  pressure 

does  a  man  of  quence  of  its  fluidity.  The  body  of  a  man  of 
common  stat-  common  stature  has  a  surface  of  about  2000 
from  the  square  inches,  whence  the  pressure,  at  15  pounds 

weight  of  the  per  square  inch,  will  be  30,000  pounds.  The 
**"'  reason  why  this  immense  weight  is  not  felt  is, 

that  the  air  within  the  body  and  its  pores  counterbalances  the 
weight  of  the  external  air.  When  the  external  pressure  is  arti- 
ficially removed  from  any  part,  it  is  immediately  felt  by  the 
reaction  of  the  internal  air. 

TPTT   .    -,  ,  521.    Heat  acts  upon  the    minute    particles 

What  effect 

has  heat  upon     of  bodies  and  forces  them  asunder,  in  opposition 

air  and  other  to  tne  attraction  of  cohesion  and  of  gravity  ;  it 
elastic  fluids  ? 

therefore  exerts  its  power  against  both  the  attrac- 

tion of  gravitation  and  the  attraction  of  cohesion.  But  as  the 
attraction  of  cohesion  does  not  exist  in  aeriform  fluids,  the 
expansive  power  of  heat  upon  them  has  nothing  to  contend  with 

*  It  has  been  computed  that  the  weight  of  the  whole  atmosphere  is  eqnal 
to  that  of  a  globe  of  lead  sixty  miles  in  diameter,  or  to  five  thousand  billions 
of  tous. 


142  NATURAL   PHILOSOPHY. 

but  gravity.  Any  increase  of  temperature,  therefore,  expands 
an  elastic  fluid  prodigiously,  and  a  diminution  of  heat  con- 
denses it. 

,tri  ,  .   JL  522.  A  column  of  air,  having  a  base  an  inch 

*\Vhat  is  the 
weight  of  a        squarej   and  reaching  to  the  top   of  the    atmo- 

column  of  air     sphere,  weighs  about  fifteen  pounds.     This  press- 

ch?   ure'    like    the    Pressure    of    li(luids>    is    exertcd 
equally  in  all  directions. 

What  is  meant  523.  The  elasticity  of  air  and  other  aeriform 

by  tlie  elasticity  fl^g  js  that  property  by  which    they  are   in- 

of  air  and  '         .   .  *    ,  .      ' 

other  aeriform  creased  or  diminished  in  extension,  according  as 

fluids  ?  they  are  compressed. 

What  effect  '  524.  This  property  exists  in  a  much  greater. 
has  an  increase  degree  in  air  and  other  similar  fluids  than  in  any 

°tionofnr ensure  other  substance-  In  fact>  Jt  has  no  known  limit » 
upon  an  aeri-  for,  when  the  pressure  is  removed  from  any  per- 
form body  ?  tion  of  air,  it  immediately  expands  to  such  a 
degree  that  the  smallest  quantity  will  diffuse  itself  over  an 
indefinitely  large  space.  And,  on  the  contrary,  when  the  press- 
ure is  increased,  it  will  be  compressed  into  indefinitely  smal 
dimensions. 

What  is  Ma-  525.  The  elasticity  or  pressure  of  air  and 
riotte's  Law  ?  a}j  gases  is  in  direct  proportion  to  their  dens- 
ity ;  or,  what  is  the  same  thing,  inversely  proportional  to 
the  space  which  the  fluid  occupies.  This  law,  which  was 
discovered  by  MariottP,  is  called  "  Mariotte's  Law" 
This  law  may  perhaps  be  better  expressed  in  the  following 
language  ;  namely,  the  density  of  an .  elastic  fluid  is  i-ti 
direct  proportion  to  the  pressure  which  it  sustains. 
IIow  does  526.  Air  becomes  a  mechanical  agent  by 

mTecMMa  means  of  its  weigH  its  elasticity,  its  inertia, 
agent  ?  and  its  fluidity. 

With  what  527.  The  'fluidity  of  air  invests  it,  as  it  invests 

power  does         %\\   other  fluids,  with  the  power  of  tran»mittinf 


TN  K  UMATICS  .  1  4  '{*, 

fluidity  invest  pressure.  But  it  has  already  been  shown,  under 
a  fluid?  the  head  of  Hydrostatics,  that  fluidity  is  a  neces- 

sary consequence  of  the  independent  gravitation  of  the  particle? 
of  a  fluid.  It  may,  therefore,  be  included  among  the  effects  of 
weight. 

528.  The  inertia  of  air  is  exhibited  in  the.  resistance  which  it 
opposes  to  motion,  which  has  already  been  noticed  under  the  head 
}f  Mechanics.*  This  is  clearly  seen  in  its  effects  upon  foiling 
bodies,  as  will  be  exemplified  in  the  experiments  with  the  air-pump 

What  is  a  529.  A  Vacuum  is  a  space  from  which  aii 

Vacuum  ?         an(j  ever  v  Other  substance  have  been  removed 

530.  The  Torricellian  vacuum  was  discovered 
Vihat  is  the       ,-,.„.         ,  i  *    •      i    •      .1      *  n      •      t 

most  perfect       Dy  Torncelh,  and  was  obtained  m  the  following 

vacuum  that       manner  :  A  tube,  closed  at  one  end,  and  about 
™  °          thirty-two  inches  long,  was  filled  with  mercury  ; 


the  open  end  was  then  covered  with  the  finger,  so 
as  to  prevent  the  escape  of  the  mercury,  and  the  tube  inverted 
and  plunged  into  a  vessel  of  mercury.  The  finger  was  then 
removed,  and  the  mercury  permitted  to  run  out  of  the  tube.  It 
was  found,  however,  that  the  mercury  still  remained  in  the  tube 
to  the  height  of  about  thirty  inches,  leaving  a  vacuum  at  the 
top  of  about  two  inches.  This  vacuum,  called  from  the  dis- 
joverer  the  Torricellian  vacuum,  is  the  most  perfect  that  has 
been  discovered.! 

*  The  fly,  as  it  is  called,  in  the  mechanism  of  a  clock  by  which  the  hours 
are  strucK,  is  an  instance  of  the  application  of  the  inertia  of  the  air  in 
Mechanics. 

t  Torricelli  was  a  pupil  of  the  celebrated  Galileo.  The  Grand  Duke  of 
Tuscany  having  had  a  deep  well  dug,  the  workmen  found  that  the  water 
would  rise  no  higher  than  thirty-two  feet.  Galileo  was  applied  to  for  an 
explanation  of  the  reason  without  success.  Torricelli  conceived  the  idea  of 
substituting  mercury  for  water,  arguing  that  if  it  was  the  pressure  of  the 
atmosphere  that  would  raise  the  water  in  the  pump  to  the  height  of  thirty- 
two  feet,  that  it  would  sustain  a  column  of  mercury  only  one-fourteenth  as 
hifh,  or  thirty  inches  only,  on  account  of  its  greater  specific  gravity.  He 
therefore  determined  to  test  it  by  experiment.  He  accordingly  filled  a 
linall  glass  tube,  about  four  feet  long,  with  mercury,  and,  stopping  the 
open  end  with  his  finger,  he  inverted  it  into  a  basin  of  mercury.  On 
removing  his  finger,  the  mercury  immediately  descended  in  the  tube,  and 
rtood  at  the  height  of  about  thirty  inches  ;  thus  demonstrating  the  fact 
that  it  was  the  pressure  of  the  air  on  the  surface  of  the  mercury  in  the  one 
•>ase,  and  of  the  water  in  the  other,  that  sustain*  •!  the  column  of  mercury 
\n  tlic  tube,  and  of  the  water  in  the  puuip. 


144  NATURAL   PHILOSOPHY. 

531.  As  this  is  one  of  the  most  important  discoveries  of  the 
science  of  Pneumatics,  it  is  thought  to  be  deserving  of  a  labored 
explanation.     The  whole  phenomenon  is  the  result  of  the  equilibrium 
of  fluids.     The  atmosphere,  pressing  by  its  weight  (fifteen  pounds 
on  every  square  inch)  on  the  surface  of  the  mercury  in  the  vessel, 
counterpoised  the  column  of  mercury  in  the  tube  when  it  was  about 
thirty  inches  high,  showing  thereby  that  a  column  of  the  atmo 
sphere  is  equal  in  weight  to  a  column  of  mercury  of  the  same  base, 
having  a  height  of  thirty  inches.     Any  increase  or  diminution  in 
the  density  of  the  air  produces  a  corresponding  alteration  in  its 
weight,  and,  consequently,  in  its  ability  to  sustain  a  longer  or  a 
shorter  column  of  mercury.     Had  water  been  used  instead  of  mer- 
cury, it  would  have  required  a  height  of  about  thirty-three  feet  to 
counterpoise  the  weight  of  the  atmospheric  column.     Other  fluids 
may  be  used,  but  the  perpendicular  height  of  the  column  of  any 
fluid,  to  counterpoise  the  weight  of  the  atmosphere,  must  be  as 
much  greater  than  that  of  mercury  as  the  specific  gravity  of  mercury 
exceeds  that  of  the  fluid  employed. 

532.  This  discovery  of  Tomcelli  led  to  the  construction  of  the 
Darometer,*  for  it  was  reasoned  that  if  it  was  the  weight  of  the 
atmosphere  which  sustained  the  column  of  mercury,  that  on  ascend- ' 
>ng  any  eminence  the  column  of  mercury  would  descend  in  pro- 
portion to  the  elevation. 

What  is  a  Ba-  533.  The  Barometer  is  an  instrument  to 
rometer?  measure  the  weight  of  the  atmosphere,  and 
thereby  to  indicate  the  variations  of  the  weather,  f 

534.  Fig.  83  represents  a  barometer.  It  ^79. 
&?Pla™  congists  Of  a  long  glass  tukej.  ^0^  thirty- 
three  inches  in  length,  closed  at  the  upper 
end  and  filled  with  mercury.  The  tube  is  then  in- 
ver  /ed  in  a  cup  or  leather  bag  of  mercury,  on  which 
the  pressure  of  the  atmosphere  is  exerted.  As  the 
tube  is  closed  at  the  top,  it  is  evident  that  the  mercury 
cannot  descend  in  the  tube  without  producing  a  vacuum. 
The  pressure  of  the  atmosphere  (which  is  capable  of 
supporting  a  column  of  mercury  of  about  thirty  inches 
in  height)  prevents  the  descent  of  the  mercury  ;  and 

*  Among  those  to  whom  the  world  is  indebted  for  the  invention  of  the 
barometer  and  its  applications  in  science,rnay  be  mentioned  the  names  of 
Descartes,  Pascal,  Morienue,  and  Boyle.  The  original  idea  is  due  to  Torri- 
telli's  experiment. 

•  t  The  word  barometer  is  from  the  Greek,  and  signifies  "a  measure  oftk< 
weight"  that  is,  of  the  atmosphere. 


PNEUMATICS. 


145 


Fig.  80. 


the  instrument,  thus  constructed,  becomes  an  implement  for 
ascertaining  the  weight  of  the  atmosphere.  As  the  air  varies 
in  weight  or  pressure,  it  must,  of  course,  influence  the  mercury 
in  the  tube,  which  will  rise  or  fall  in  exact  proportion  with  the 
pressure.  When  the  air  is  thin  and  light,  the  pressure  is  less, 
and  the  mercury  will  descend  ;  and,  when  the  air  is  dense  and 
heavy  the  mercury  will  rise.*1  At  the  side  of  the  tube  there 
is  a  scale,  marked  inched  and  tenths  of  an  inch,  to  note  the  rise 
and  fall  of  the  mercury. 

535.  The  barometer,  as  thus  constructed,  only  required  the 
addition  of  an  index  and  a  weather-glass,  as  seen 
in  Fig.  80,  tc  give  a  fair  and  true  announcement 
of  the  state  and  weight  of  the  atmosphere.  The 
instruments  are  now  manufactured  in  several  dif- 
ferent forms.  The  different  forms  of  the  barometer 
in  general  use  are  the  common  Mercurial  Barom- 
eter, the  Diagonal,  and  the  Wheel  Barometer,  all 
of  which  are  constructed  with  a  column  of  mer- 
cury. The  Aneroid  or  Portable  Barometer  is  a 
new  instrument,  in  which  confined  air  is  substi- 
tuted for  mercury.  This  is  a  convenient  form  of 
the  instrument  for  portable  purposes.  But  the 
principle  is  the  same  in  all,  and  repeated  observa- 
tions during  the  ascent  of  the  loftiest  mountains 
in  Europe  and  America  have  confirmed  the  truth 
of  barometrical  announcements ;  for,  by  its  indi- 
cations, the  respective  heights  of  the  acclivities  in 
high  regions  can  now  be  ascertained  by  means  of 
this  instrument  better  than  by  any  other  course, 
—with  this  advantage,  too,  that  no  proportionate 
height  need  be  known  to  ascertain  the  altitude.! 

*  The  elasticity  of  the  air  causes  an  increase  or  diminution  of  its  bulk, 
according  as  it  is  affected  by  heat  and  cold;  and  this  increase  and  diminu- 
tion of  bulk  materially  affect  its  specific  gravity.  The  height  of  a  column 
of  mercury  that  can  be  sustained  by  a  column  of  the  atmosphere  must, 
therefore,  be  affected  by  the  state  of  the  atmosphere. 

t  From  the  explanation  whieh  has  now  been  given  of  the  barometer,  it 


146  NATURAL  PHILOSOPHY. 

On    hat  ^^'    ^e  Pressure  °f  *^e  atmosphere  on  th<§ 

principle  is        mercury,  in  the  bag  or  cup  of  a  barometer,  being 

the  barometer  exerted  on  the  principle  of  the  equilibrium  of 
constructed? 

fluids,  must  vary  according  to  the  situation  in 

which  the  barometer  is  placed.  For  this  reason,  it  will  be  the 
greatest  in  valleys  and  low  situations,  and  least  on  the  top  of 
high  mountains.  Hence  the  barometer  is  often  used  to  ascer- 
tain the  height  of  mountains  and  other  places  above  the  level  of 
the  sea. 

Wlien  is  the       ^37.    The  air  is  the  heaviest  in  dry  weather, 

atmosphere        and    consequently   the     mercury   will   then   rise 

highest.     In  wet  weather  the  dampness   renders 

will  readily  be  seen  that  a  column  of  any  other  fluid  will  answer  as  well  as 
mercury,  provided  the  tube  be  extended  in  an  inverse  proportion  to  the 
specific  gravity  of  the  fluid.  But  mercury  is  the  most  convenient,  because 
it  requires  the  shortest  tube. 

In  navigation  the  barometer  has  become  an  important  element  of 
guidance,  and  a  most  interesting  incident  is  recounted  by  Captain  Basil 
liall,  indicative  of  its  value  in  the  open  sea.  While  cruising  off  the  coast 
of  South  America,  in  the  Medusa  frigate,  one  day,  when  within  the  tropics 
the  commander  of  a  brig  in  company  was  dining  with  him.  After  dinner 
Vthe  conversation  turned  on  the  natural  phenomena  of  the  region,  when 
.'Captain  Hali's  attention  was  accidentally  directed  to  the  barometer  in  the 
*tate-room  where  they  were  seated,  and,  to  his  surprise,  he  observed  it  to 
'  evince  violent  and  frequent  alteration.  His  experience  told  him  to  expect 
bad  weather,  and  he  mentioned  it  to  his  friend.  His  companion,  however, 
only  laughed,  for  the  day  was  splendid  in  the  extreme,  the  sun  was  shining 
with  its  utmost  brilliance,  and  not  a  cloud  specked  the  deep-blue  sky 
above.  But  Captain  Hall  was  too  uneasy  to  be  satisfied  with  bare  appear- 
ances. He  hurried  his  friend  to  his  ship,  and  gave  immediate  directions 
for  shortening  the  top  hamper  of  the  frigate  as  speedily  as  possible.  His 
lieutenants  and  the  men  looked  at  hiir,  in  mute  surprise,  and  one  or  two  of 
the  former  ventured  to  suggest  the  inucility  of  the  proceeding.  The  cap- 
tain, however,  persevered.  The  sails  were  furled,  the  top-masts  were 
struck; -in  short,  everything  thi?,t  could  oppose  the  wind  was  made  as 
snug  as  possible.  His  friend,  on  the  contrary,  stood  in  under  every  sail. 

The  wisdom  of  Captain  Hall's  proceedings  was,  however,  speedily  evi- 
dent ;  just,  indeed,  as  he  was  beginning  to  doubt  the  accuracy  of  his  in- 
strument. For  hardly  had  the  necessary  preparations  been  made,  and 
while  his  eye  was  ranging  over  the  vessel  to  see  if  his  instructions  had  been 
obeyed,  a  dark  hazy  hue  was  seen  to  rise  in  the  horizon,  a  leaden  tint 
rapidly  overspread  the  sullen  waves,  and  one  of  the  most  tremendous  hur- 
ricanes burst  upon  the  vessels  that  ever  seaman  encountered  on  his  ocean 
home.  The  sails  of  the  brig  were  immediately  torn  to  ribbons,  her  masts 
went  by  the  board,  and  she  was  left  a  complete  wreck  on  the  tempestuous 
surf  which  raged  around  her,  while  the  frigate  was  driven  wildly  along  at  a 
furious  rate,  and  had  to  scud  under  bare  poles  across  the  wide  Pacific,  full 
three  thousand  miles,  before  it  could  be  said  that  she  was  in  safety  from  the 


PNEUMATICS.  147 

the  air  less  salubrious,  and  it  appears,  therefore,  more  heavy 
then,  although  it  is,  in  fact,  much  lighter. 
A.  what  time        ^38.     The  greatest  depression  of  the  barometer 
of  the  day  is    occurs  daily  at  about  four  o'clock,  both  in  the  morn- 

'and  lowest  *n&  an(^  *n  *k°  aftem0011 5  an^  ^8  highest  elevation 
state  of  the  at  about  ten  o'clock,  morning  and  night.  In  sun* 
barometer  ?  mer  these  extreme  points  are  reached  an  hour  or 
two  earlier  in  the  morning,  and  as  much  later  in  the  afternoon, 

589.  Rules  have  been  proposed  by  which  the  changes  of  th« 
weather  may  be  predicted  by  means  of  the  barometer.  Heno« 
the  graduated  edge  of  the  instrument  is  marked  with  the  words 
"ram,"  "fair,"  "changeable."  "frost,"  .&c.  These  expressions 
are  predicated  on  the  assumption  that  the  changes  of  the  weathsr 
may  correctly  be  predicted  by  the  absolute  height  of  the  mercury.- 
But  on  this  little  reliance  can  be  placed.  The  best  authorities  agree 
that  it  is  rather  the  change  in  the  height  on  which  the  predications 
must  be  made. 

540.  As  the  barometer  is  much  used  at  the  present  day,  it  hag 
been  thought  expedient  to  subjoin  a  few  general  and  special  rules, 
from  different  authorities,  by  which  some  knowledge  of  the  uses  of 
the  instrument  may  be  acquired. 

541.  General  Rules  by  which   Changes  of  the    Weather  may  be  prognost* 

cated  by  means  of  the  Barometer.* 

(1.)  Generally  the  rising  of  the  mercury  indicates  the  approach  of  fair 
weather. 

(2.)  In  sultry  weather  the  fall  of  the  mercury  indicates  coming  thunder 
In  winter  the  rise  of  the  mercury  indicate?  frost  In  frost,  its  fall  indicates 
thaw,  and  its  rise  indicates  snow. 

(3.)  Whatever  change  of  weather  suddenly  follows  a  change  in  the 
barometer,  may  be  expected  to  last  but  a  short  time.  Thus,  if  fair  weather 
follow  immediately  the  rise  of  the  mercury,  there  will  be  very  little  of  it,, 
and,  in  the  same  way,  if  foul  weather  follow  the  fall  of  the  mercury,  it  will 
last  but  a  short  time. 

'4.)  If  fair  weather  continue  for  several  days,  during  which  the  mercury 
continually  falls,  along  succession  of  foul  weather  will  probably  ensue;  and 
again,  if  foul  weather  continue  for  several  days,  while  the  mercury  con- 
tinually rises,  a  long  succession  of  fair  weather  will  probably  succeed. 

(5.)  A  fluctuating  and  unsettled  state  in  the  mercurial  column  indicate* 
changeable  weather.  —  Lardner,  page  75,  Pneumatics. 

542.  Special  Rules  ly  which  we  may  know  the.  Changes  of  the  Weather  ly 

means  of  the  Barometer^ 

(1.)  The  barometer  is  highest  of  all  during  a  long  frost,  and  it  generally 
rises  with  a  north-west  wind. 

*  These  rules,  says  Dr.  Lardner,  from  whose  work  they  are  extracted, 
may  to  some  extent  be  relied  upon,  but  they  are  subject  to  some  uncer- 
tainty. 

t  These  rules  are  from  a  different  authority.  ^ 


148  NATURAL   PHILOSOPHY. 

(2  )  The  barometer  is  lowest  of  all  during  a  thaw  which  follows  a  long 
frost,  and  it  generally  falls  with  a  south  or  east  wind. 

(3.)  While  the  mercury  in  the  barometer  stands  above  30°  vhe  air  must 
oe  very  dry  or  very  cold,  or  perhaps  both,  and  no  rain  may  be  ixpected. 

(4.)  When  the  mercury  stands  very  low  indeed,  there  will  never  be  much 
rain,  although  a  fine  day  will  seldom  occur  at  such  times. 

(5.)  In  summer,  after  a  long  continuance  of  fair  weather,  the  barometer 
#ill  fall  gradually  for  two  or  three  days  before  rain  falls  ;  but,  if  the  fall 
of  the  mercury  be  very  sudden,  a  thunder-storm  may  be  expected. 

(G.)  When  the  sky  is  cloudless  and  seems  to  promise  fair  weather,  if  the 
barometer  is  low,  the  face  of  the  sky  will  soon  be  suddenly  overcast. 

(7.)     Dark,  dense  clouds  will  pass  over  without  rain  when  the  barometer 
is  high  ;  but  if  the  barometer  be  low  it  will  often  rain  without  any  appeal 
ance  of  clouds.  « 

(8.)     The  higher  the  mercury,  the  greater  probability  of  fair  weather. 

(9.)  When  the  mercury  is  in  a  rising  state,  fine  weather  is  at  hand  ;  but 
wrhen  the  mercury  is  in  a  falling  state,  foul  weather  is  near. 

(10.)  In  frosty  weather,  if  snow  falls,  the  mercury  generally  rises  to 
30J,  where  it  remains  so  long  as  the  snow  continues  to  fall;  if  after  this  the 
weather  clears  up,  very  severe  cold  weather  may  be  expected. 

It  will  be  observed  that  the  barometer  varies  more  in  winter  than  in 
Bummer.  It  is  at  the  highest  in  May  and  August;  then  in  June,  March, 
September  and  April.  It  is  the  lowest  in  November  and  February;  then  in 
October,  July,  December  and  January. 

[These  rules  are  from  Dr.  Brewer's  work  called  "  The  Science  of  Familiar 
Things.'*] 

543.  OP  THE  DIFFERENT  STATES  OF  THE  BAROMETER.  —  Of  the  Fall  of  tk« 
Barometer,  —  In  very  hot  weather  the  fall  of  the  Barometer  indicates  thun- 
ier.  Otherwise,  the  sudden  fall  of  the  barometer  leads  to  the  expectation 
»)f  high  wind. 

In  frosty  weather  the  fall  of  the  barometer  denotes  a  thaw. 

If  wet  weather  follow  soon  after  the  fall  of  the  barometer,  but  little  oi 
«uch  weather  may  be  expected. 

In  wet  weather,  if  the  barometer  falls,  expect  much  wet. 

In  fair  weather,  if  the  barometer  falls  and  remains  low,  expect  much  wet 
In  a  few  days,  and  probably  wind. 

The  barometer  sinks  lowest  of  all  for  wind  and  rain  together;  next  to 
that  for  wind,  except  it  be  an  east  or  north-east  wind. 

54i.  Of  the  Rise  of  the  Barometer.  —  In  winter  the  rise  of  the  barometer 
presages  frost. 

In  frosty  weather,  the  rise  of  the  barometer  presages  snow. 

If  fair  weather  happens  soon  after  the  rise  of  the  barometer,  expect  but 
little  of  it. 

In  wet  weather,  if  the  mercury  rises  high  and  remains  so,  expect  continued 
fine  weather  in  a  day  or  two. 

In  wet  weather,  if  the  mercury  rises  suddenly  very  high,  fine  weather 
will  not  last  long. 

The  barometer  rises  highest  of  all  for  north  and  west  winds;  for  all  other 
winds,  it  sinks. 

645.  The  Barometer  in  an  Unsettled  State.  If  the  motion  of  the  mercury 
b*  unsettled,  expect  unsettled  weather. 


PNEUMATICS.  149 

If  it  stand  at  "much  rain"  and  rise  to  "changeable,"  expect  fair  weather 
of  short  continuance. 

If  it  stand  at  "fair"  and  fall  to  "changeable,"  expect  foul  weather. 

Its  motion  upwards  indicates  the  approach  of  fine  weather  j  its  motion 
downward  indicates  the  approach  of  foul  weather. 

Wlt.at  is  ike  ^46.  THE  THERMOMETER.  —  The  Ther- 
1 Thermometer,  mometer  *  is  an  instrument  to  indicate  the  tem- 
^rindple  is  it  perature  of  the  atmosphere.  It  is  constructed 
Constructed?  On  the  principle  that  heat  expands  and  cold 
contracts  most  substances. 

547.  The  thermometer  consists  of  a  capillary  tube,  closed  at 
the  top  and  terminating  downwards  in  a  bulb.  It  is  filled  with 
mercury,  which  expands  and  fills  the  whole  length  of  the  tube  or 
contracts  altogether  into  the  bulb,  according  to  the  degree  of 
heat  or  cold  to  which  it  is  exposed.  Any  other  fluid  may  be 
used  which  is  expanded  by  heat  and  contracted  by  cold,  instead 
of  mercury.  Fig.  81 

b48.  On  the  side  of  the  thermometer  is  a  scale  to  ^\ 
indicate  the  rise  and  fall  of  the  mercury,  and  conse- 
quently the  temperature  of  the  weather. 
WJiat  scale  is  ^49.  There  are  several  different  scales 
adopted  for  the  applied  to  the  thermometer,  of  which  those 
^ihis^oun  of  ^ahrenbeit,  Reaumur,  Delisle  and  Gel- 
fry  *  sius,  are  the  principal.  The  thermometer 
in  common  use  in  this  country  is  graduated  by  Fahren- 
neit's  scale,  which,  commencing  with  0,  or  zero,  extends 
upwards  to  212  degrees,  the  boiling  point  of  water,  and 
downwards  to  20  or  30  degrees.  The  scales  of  Reau- 
mur and  Celsius  fix  zero  at  the  freezing  point  of  water  ; 
and  that  of  Delisle  at  the  boiling  point. 

What  is  the  550.  THE  HYGROMETER.  —  The  Hygrom- 
Hygromete*-  ?  eter  is  an  instrument  for  showing  the  degree 
of  moisture  in  the  atmosphere. 

*  The  word  "Thermometer"  is  from  the  Greek,  and  means  "a  meaaurt 
of  heat."  "  Hygrometer  "  means  "a  measure  of  moisture." 


150  NATURAL    PUTT/)SOPUY. 

How  is  it  con-  551.  The  hygrometer  may  be  constructed  of 
tt7-u$ted  ?  any  material  which  dryness  or  moisture  expand* 
or  contracts ;  such  as  most  kinds  of  wood,  catgut,  twisted  cord, 
the  beard  of  wild  oats,  &c.  It  is  sometimes  also  composed  of  a 
scale  balanced  by  weights  on  one  sMe,  and  a  sponge,  or  ot1  er 
substance  which  readily  imbibes  moisture,  on  the  other. 

552.  By  the  action  of  the  sun's  heat  upon  the  surface  of  the 
earth,  whether  land  or  water,  immense  quantities  of  vapor  are  raised 
into  the  atmosphere,  supplying  materials  for  all  the  water  which  is 
deposited  again  in  the  various   forms  of  dew,  fog,  rain,  snow,  and 
bail.     Experiments  have  been  made  to  show  the  quantity  of  moist- 
ure thus  raised  from  the  ground  by  the  heat  of  the  sun.     Dr.  Wat- 
son found  that  an  acre  of  ground,  apparently  dry  and  burnt  up  by 
the  sun,  dispersed  into  the  air  sixteen  hundred  gallons  of  water  in 
the  space  of  twelve  hours.    His  experiment  was  thus  made  :  He  put 
a  glass,  mouth  downwards,  on   a   grass-plot,  on  which   it  had  net 
rained  for  above  a  month.     In  less  than  two  minutes  the  inside  was 
covered  with  vapor  ;  and  in  half  an  hour  drops  began  to  trickle  down 
its  inside.     The  mouth  of  the  glass  was  20  square  inches.     There 
are  12%  square  inches  in  a  square  yard,  and  4840  square  yards  in 
,m  acre.     When  the  glass  had  stood  a  quarter  of  an  hour,  he  wiped 
it  with  a  piece  of  muslin,  the  weight  of  which  had  been  previously 
ascertained.     When  the  glass  had  been  wiped  dry,  he  again  weighed 
the  muslin,  and  found  that  its  weight  had  increased  six  grains  by 
the  water  collected  from  20  square  inches  of  earth  ;  a  quantity  equa' 
to  1600  gallons, -from  an  acre,  in  12  Lours.     Another  experiment, 
after  rain  had  fallen,  gave  a  much  Larger  quantity. 

553.  When  the  atmosphere  is  colder  than  the  earth,  the  vapor 
which  arises  from  the  ground,  or  a  body  of  water,  is  condensed  and 
becomes  visible.     This  is  the  way  that  fog  is  produced.     When  the 
darth  is  colder  than  the  atmosphere,  the  moisture  in  the  atmosphere 
condenses  in  the  form  of  dew,  on  the   ground,  or   other  surfaces. 
Clouds  are  nothing  more  than  vapor  condensed  by  the  cold  of  the 
upper  regions  of  the  atmosphere.     Rain  is  produced  by  the  sudden 
cooling  of  large  quantities  of  watery  vaj>or.     Snow  and  hail  are 
produced  in  a  similar  manner,  and  differ  from  rain  only  in  the  de- 
gree of  cold  which  produces  them. 

What   is   the  554.    THE  DlVER;S  BELL  OR  DlVING-BELL. 

Diving-bell,      —  The  Diving-bell  is  a  large  vessel  shaped  like 

and  on  what  .  ,  b  , .  f.  . 

principle  is  it    an  inverted  goblet,  in  which  a  person  may 

constructed?  safely  descend  to  great  depths  in  the  water. 
It  is  constructed  on  th'e  principle  of  the  impenetrability  of 
air. 


PNEUMATICS. 


151 


555.  It  has  already  been  stated  that  air,  being  a  material  sub 
stance,  possesses  all  the  given  essential  properties  of  -»wcer,  and 
among  them  the  property  of  impenetrability.     The  weight  of  the 
air  giving  it  a  pressure  in  every  direction,  or  the  property  of  fluidity, 
it  penetrates  and  fills  all  things  around  us,  unless  by  mechanical 
means  it  be  carefully  excluded.     An  open  vessel,  of  whatever  kind, 
is  always  full  either  of  air  or  of  some  other  substance,  and  unless 
the  air  is  first  permitted  to  escape  no  other  substance  can  take  the 
place  of  the  air. 

556.  If  a  tumbler  be  inverted  and  immersed  in  water,  the  water 
will  not  rise  in  the  tumbler,  because  the  air  in  the  tumbler  fills  it. 
[f  the  tumbler  be  inclined  so  as  to  let  the  air  ascend  in  obedience  to 
the  laws  of  the  equilibrium  of  fluids,  the  water  will  rush  in  and  dis- 
place the  air,  while  the  lighter  air.  ascending,  rises  to  the  surface  of 
the  water.     If  this  experiment  be  made  with  a  bottle,  the  air  will 
rise  in  bubbles  with  a  gurgling  sound.     The  same  experiment  may  be 
made  with  a  tube  closed  at  one  end  by  the  finger ;  the  water  will  not 
enter  the  tube  until  bv  the  removal  of  the  finger  the  air  be  permitted 
to  escape.    It  is  on  this  principle  that  the  diving-bell  is  constructed 

557.     Fig.  82  represents  a  Fig>  82' 

Explain  the  con-   ...       ...    °  .          _ 

struction  of  the  diving-bell.    It  consists  of  a 

diving-bell  by      large  heavy  vessel,  formed 
Fi.  82. 


of  various  shapes),  with  the  mouth  open.  It 
descends  into  the  water  with  its  mouth  down- 
wards. The  air  within  it  having  no  outlet, 
it  is  compelleu  by  the  order  of  specific  grav- 
ities to  ascend  in  the  bell,  and  thus  (as  water 
and  air  cannot  occupy  the  same  space  at  the 
same  time)  prevents  the  water  from  rising 
in  the  bell.  A  person,  therefore,  may  de- 
scend with  safety  in  the  bell  to  a  great  depth 
in  the  sea,  and  thus  recover  valuable  articles 
that  have  been  lost.  A  constant  supply  of 
fresh  air  is  sent  down,  either  by  means  of 
barrels,  or  by  a  forcing-pump.  In  the  Fig. 
P  represents  the  bell  with  the  diver  in  it. 
rif  tube  attached  to  one  side  and  reaching  the  air  within ;  and 
P  is  the  forcing-pump  through  which  air  is  forced  into  the  bell. 
The  forcing-pump  is  attached  to  the  tube  by  a  joint  at  D.  When 
die  bell  descends  to  a  great  depth,  the  pressure  of  the  water 


C  is  a  bent  metal- 


NATURAL   PHILOSOPHY. 


condenses  the  air  within  the  bell,  and  causes  the  water  to  asoen;! 
in  the  bell.  This  is  forced  out  bv  constant  accessions  of  fresh 
air,  supplied  as  above  mentioned.  Great  care  mast  be  taken 
that  a  constant  supply  of  fresh  air  is  sent  down,  otherwise  the 
lives  of  those  within  the  bell  will  be  endangered.  The  heated 
and  impure  air  is  allowed  to  escape  through  a  stop-cock  in  the 
upper  part  of  the  bell.  (See  par.  1462.) 

558.  THE    COMMON    WATER    PUMP.— 

Hew    is    water    ,Tr  ^        .          .      ,    .      ,,  , 

raised  in  a  com-    Water  is  raised  in"  the  common  pump  by 

mon  pump 


How  high  may 
water  be  raised 
by    a 
pump  ? 


Fig.  83. 


means   of  the   pressure  of  tho   atmosphere 
on   the   surface   of  the  water.     A  vacuum 
common    being   produced   by   raising   the   piston   or 
pump-box,*    the   water   below   is      Fig.  83. 
forced  up  by  the  .atmospheric  pressure,   on  the 
principle  of  the  equilibrium  of  fluids.     On  this 
principle  the  water  can    be   raised  only  to  the 
height  of  about  thirty-three   feet,   because   the 
pressure  of  the  atmosphere  will  sustain  a  column 
of  water  of  that  height  only. 

559.  Fig.  63  represents  the  common 
pump,  generally  called  the  suction- 
pump.  The  body  consists  of  a  large  tube, 
or  pipe,  the  lower  end  of  which  is  immersed  in  the 
water  which  it  is  designed  to  raise.  P  is  the  piston, 
V  a  valve  t  in  the  piston,  which,  opening  upwards, 
admits  the  water  to  rise  through  it,  but  prevents  its 
return.  Y  is  a  similar  valve  in  the  bodj.  of  the 

*  In  order  to  produce  such  a  vacuum,  it  is  necessary  that  the  piston  01 
box  should  be  accurately  fitted  to  the  bore  of  the  pump  ;  for,  if  the  air 
above  the  piston  has  any  means  of  rushing  in  to  fill  the  vacuum,  as  it  i* 
produced  by  the  raising  of  the  piston,  the  water  will  not  ascend  The  pis 
ton  is  general'j  worked  by  a  lever,  which  is  the  handle  of  the  pump,  not 
represented  in  the  figure. 

f  A  valve  is  a  lid,  or  cover,  so  contrived  as  to  open  a  communication  ic 
one  way  and  close  it  in  the  other.  Valves  are  made  in  different  ways^ 
according  to  the  use  for  which  they  are  intended.  In  the  common  pumj 
they  are  generally  made  of  thick  leather  partly  covered  with  wood  I.i 
the  air-pump  they  are  made  of  oiled  silk,  or  thin  leather  softened  wit* 
oil.  The  clapper  of  a  pair  of  bellows  is  a  familiar  specimen  "i  »  v<»V« 
The  valves  of  a  pump  are  commonly  called  b-.txta 


PNEUMATICS.'  153 

pump,  below  the  piston.  When  the  pump  is  not  in  action,  the 
Calves  are  closed  by  their  own  weight ;  but  when  the  piston  is 
raised  it  draws  up  the  column  of  water  which  rested  upon  it 
producing  a  vacuum  between  the  piston  and  the  lower  valve  Y 
The  water  below  immediately  rushes  through  the  lower  valve 
and  fills  the  vacuum.  When  the  piston  descends  a  second  time, 
the  water  in  the  body  of  the  pump  passes  through  the  valve 
V,  and  on  the  ascent  of  the  piston  is  lifted  up  by  the  piston, 
and  a  vacuum  is  again  formed  below,  which  is  immediately 
filled  by  the  water  rushing  through  the  lower  valve  Y.  In 
this  manner  the  body  of  the  pump  is  filled  with  water,  until  it 
reaches  the  spout  S,  where  it  runs  out  in  an  uninterrupted  stream. 

560.  In  the  description  here  given  of  the  common  pmnp,  as 
well  as  in  the  figure,  it  will  be  observed  that  the  common  form 
of  the  handle  of  the  pump  is  not  noticed.    The  handle  of  the  pump 
is  merely  a  lever  of  the  first  kind ;  the  fulcrum  is  the  pin  which 
attaches  it  to  the  pump,  and  the  iron  rod  connected  with  the 
upper  valve  of  the  pump  is  raised  or  depressed  by  means  of  the 
handle. 

561.  Although  water  can  be  raised  by  the  atmospheric  pressure 
only  to  the  height  of  thirty-three  feet  above  the  surface,  the  com- 
mon pump  is  so  constructed  that  after  the  pressure  of  the  atmos- 
phere has  forced  the  water  through  the  valve  in  the  body  of  the 
pump,  and  the  descent  of  the  piston  has  forced  it  through  the  valve 
in  the  piston,  it  is  lifted  up,  when  the  piston  is  raised.     For  this 
reason,  this  pump  is  sometimes  called  the  lifting  pump.     The  dis- 
tance of  the  upper  valve  from  the  surface  of  the  water  must  never 
exceed  thirty-two  feet ;  and  in  practice  it  must  be  much  less. 

562.  THE  FORCING-PUMP.    The  Forcing- 

How   does    the  T/V         r«  ,1 

Forcing-pump     pump   differs   from   the   common   pump    in 

differ  from  the    having  a  forcing  power  added,  to  raise  the 

common  pump?  .      «,'.«. 

water  to  any  desired  height. 

563.    Fig.  84  represents  the  forcing  -  pump.     The 
body  and  lower  valve  V  are  similar  to  those  in  the 
common  pump.     The  piston  P  has  no  valve,  but  is 
solid;    when,   therefore,   the  vacuum   is   produced  above  the 


'54 


NATURAL    PHILOSOPHY. 


pl£-84  lower  valve,  the  water,  on  the  dcationt 

of  the  piston,  is  forced  through  the  tube 
into  the  reservoir  or  air-vessel  B,  where 
it  compresses  the  air  above  it.  The  air, 
by  its  elasticity,  forces  the  water  out 
through  the  jet  J  in  a  continued  stream, 
and  with  great  force.  It  is  on  this  prin- 
"TT"  }}  ciple  that  fire-engines  are  constructed. 

TL**^'        Sometimes  a  pipe  with  a  valve  in  it  is 
1 1          S  substituted  for  the  air-vessel ;  the  water 

k-^  is  then  thrown  out  in  a  continued  stream, 

but  not  with  so  much  force. 


How  &  the 

Fire-engine 
constructed  f 


564.  THE  FIRE-ENGINE  consists  of  two  forcing- 
pumps,  worked  successively  by  the  elevation  and 
depression  of  two  long  levers  of  the  second  kina, 
called  "  Brakes." 

Tig.  85. 


565.  THE  AIR-PUMP. —  The  Air-pump  13 
a  machine  constructed  on  the  principle  of  the 

and'on  what ^     elasticity  of  the  air,  for  the  purpose  cf  ex 

constructed  f  nausting  the  air  from  a  vessel  prepared  for 
the  purpose.  This  vessel  is  called  a  receiver. 

and  is  made  of  glass,  in  order  that  the  effects  of  the  removal 

of  the  air  may  be  seen. 

566.  Air-pumps  are  made  in  a  great  variety  of  forms ;  but  all 
are  constructed  on  the  principle  that,  w,r  en  any  portion  of  confined 


PNEUMATICS. 


155 


air  is  removed,  the  risudue,  immediately  expanding,  by  its  elasticity 
fills  the  space  occupied  by  the  portion  that  has  been  withdrawn. 


Explain  the  con- 


Fig.  86. 


567.  Fig.  86  represents  a  single-barrel  air 
Ttruciion  of  the  pump,  used  both  for  condensing  and  exhausting. 
air-pump  by  A  D  is  the  stand  or  platform  of  the  instru- 
ment, which  is  screwed  down  to  the  table  by 
means  of  a  clamp,  underneath,  HCQ> 
which  is  not  represented  in  the 
figure.  R  is  the  glass  vessel, 
or  bulbed  receiver,  from  which 
the  air  is  to  be  exhausted.  P 
is  a  sol*d  piston,  accurately  fit- 
ted to  the  bore  of  the  cylinder, 
and  H  the  handle  by  which  it 
is  moved.  The  dotted  line  T 
represents  the  communication 
between  the  receiver  H  and  the 
barrel  B ;  it  is  a  tub  hrough 

which  the  air,  entering  at  the  opening  I,  on  the  plate  of  the 
pump,  passes  into  the  barrel  through  the  exhausting  valve  E  v. 
c  v  is  the  condensing  valve,  communicating  with  the  barrel  B 
by  means  of  an  aperture  near  E,  and  opening  outwards  through 
the  condensing  pipe  p. 

Explain  the  op-  568'  The  operation  of  the  pump  is  as  follows  • 
eration  of  the  The  piston  P  being  drawn  upwards  by  the  han- 
air-pump  by  ,jle  H,  the  air  in  the  receiver  R,  expanding  bv 
its  elasticity,  passes  by  the  aperture  I  througt 
the  tube  T,  and  through  the  exhausting  valve  E  v,  into  the  bar- 
rel. On  the  descent  of  the  piston,  the  air  cannot  return  through 
that  valve,  because  the  valve  opens  upwards  only :  it  must, 
therefore,  pass  through  the  aperture  by  the  side  of  the  valve, 
and  through  the  condensing  valve  c  v,  into  the  pipe  j9,  where  it 
passes  out  into  the  open  air.*  It  cannot  return  through  the  con- 
densing valve  c  v,  because  that  valve  opens  outwards  only.  By 
continuing  this  operation,  every  ascent  and  descent  of  the  piston 
P  must  render  the  air  within  the  receiver  B.  more  and  wore 
7 


156  NATURAL    PHILOSOPHY. 

rare,  until  its  elastic  power  is  exhausted.  Tne  receiver  L<  tbeo 
said  to  be  exhausted;  and,  although  it  stiii  contains  a  smaiJ 
quantity  of  air,  yet  it  is  in  so  rare  a  state  that  the  space  within 
the  receiver  is  considered  a  vacuum. 

569.  Prom  this  statement  it  will  appear  that  a  perfect  vacuum 
can  never  be  obtained  by  the  air-pump  as  at  present  constructed. 
But  so  much  of  the  air  within  a  receiver  may  be  exhausted  that  the 
residue  will  be  reduced  to  such  a.  degree  of  rarity  as  to  subserve 
most  of  the  practical  purposes  of  a  vacuum.  The  nearest  approach 
made  to  a  perfect  vacuum  is  the  famous  experiment  of  Torricelli, 
which  has  been  explained  in  No.  530.  That  would  be  a  perfect 
vacuum,  were  there  not  vapor  rising  from  the  mercury. 

570.  Prom  the  -explanation  which  has  been 
&be  cwLfed  Siven  of  the  operation  of  this  air-pump,  it  will 
by  means  tf  the  readily  be  seen  that,  by  removing  the  receiver 
pump  which  has  ft  an(j  screwing  any  vessel  to  the  pipe  p,  the 
l>een  described?  .  J  m, 

air  may  be  condensed  in  the  vessel.     Thus  the 

pump  is  made  to  exhaust  or  to  condense,  without  alteration. 

(Vtifii  is  a  con-  *^1.  Air-pumps  in  general  are  not  adapted 
densing  syr-  for  3ondensation  ;  that  office  being  performed  by 
tnSe  •  an  instrument  called  "  a  condensing  syringe," 

which  is  an  air-pump  reversed^  its  valves  being  so  arranged  as 
to  force  air  into  a  chamber,  instead  of  drawing  it  out.  For 
this  purpose,  the  valves  open  inwards  in  respect  to  the  chamber, 
while  in  air-pumps  they  open  outwards. 

572.  A  guage,  constructed  on  the  principle  of  the  barometer,  ia 
sometimes  adjusted  to  the  air-pump,  for  the  purpose  of  exhibiting 
ihe  degree  of  exhaustion. 


How    does    the         ^^'  ^ne  Double  air-pump  differs  from  tho 
double  air-pump    single  air-pump,  in  having  two  barrels  and  two 

differ  from  the    pjgtons  ;  which,  instead  of  being  moved  by  the 
single  ? 

hand,  are  worked  by  means  oi  a  toothed  wheel, 

pla-ying  in  notches  of  the  piston-rods. 

Fig.  87  represents  an  air-pump  of  a  different  construction.  In 
this  pump  the  piston  is  stationary,  while  motion  is  given  to  the 
barrel  by  means  of  the  lever  H.  The  barrel  is  kept  in  a  proper 
position  by  means  of  polished  steel  guides. 


PNEUMATICS. 


151 


Fig.  87. 


574.  By  means  of  the  air-pump  many  interesting  experiments 
may  be  performed,  illustrating  the  gravity,  elasticity,  fluidity,  *and 
inertia  of  air. 

575.  EXPERIMENTS  ILLUSTRATING  THE   GRAVITY  OP   AIR.- -Having 
adjusted  the  receiver  to  the  plate  of  the  air-pump,  exhaust  the  air 
end  the  receiver  will  be  held  firmly  on  the"  plate.     The  forcn  which 
confines  it  is  nothing  more  than  the  weight  of  the  external  air 
which,  having  no  internal  pressure  to  contend  with,  presses  with  a 
force  of  nearly  fifteen  pounds  on  every  square  inch  of  the  external 
surface  of  the  receiver. 

576  The  exact  amount  of  pressure  depends  on  the  degree  or  ex- 
haustion, being  at  its  maximum  of  fifteen  pounds  when  there  is  a 
perfect  vacuum.  On  readmitting  the  air,  the  receiver  may  be  readily 
*enioved.* 

577.  THE  MAGDEBURGH  CUPS,  OR  HEMI- 
SPHERES.—  Fig.  88  represents  the  Magdeburgb 
Cups,  or  Hemispheres.  They  consist  of  two  hol- 
low brass  cups,  the  edges  of  which  are  accu- 
rately fitted  together.  They  each  have  a  handle, 

*  The  air  is  readmitted  into  the  receiver  by  turning  a  screw  which  is  in- 
serted into  the  receiver,  in  which  there  is  an  aperture,  through  which  the 
external  air  rushes  with  considerable  force. 


What   are    the 
Magdeburgh 
Cups,  and  what 
Jo    they     illus- 
trate ? 


158 


NATURAL   PHILOSOPHY. 


**  88-  to  one  of  which  a  stop-cock  is  fitted.  The  stop- 
cock, being  attached  to  one  of  the  cups,  is  to  be 
screwed  to  the  plate  of  the  air-pump,  and  left 
open.  Having  joined  the  other  cup  to  that  on 
the  pump,  exhaust  the  air  from  within  them, 
turn  the  stop-cock  to  prevent  its  readmission, 
and  screw  the  handle  that  had  been  removed  to 
the  stop-cock.  Two  persons  may  then  attempt 
to  draw  the  cups  asunder.  It  will  be  found  that 
great  power  is  required  to  separate  them ;  but, 
on  readmitting  the  air  between  them,  by  turning 
the  cock,  they  will  fall  asunder  by  their  own 
weight.  When  the  air  is  exhausted  from  within  them,  the  press- 
ure of  the  surrounding  air  upon  the  outside  keeps  them  united. 
This  pressure  being  equal  to  a  pressure  of  fifteen  pounds  on  every 
square  inch  of  the  surface,  it  follows  that  the  larger  the  cups; 
or  hemispheres,  the  more  difficult  it  will  be  to  separate  them. 

578.  The  Magdeburg  Cups  derive  their  name  from  the  city 
where  the  experiment  was  first  attempted.  Otto  Guericke  con- 
structed two  hemispheres  which,  when  the  air  was  exhausted,  were 


helc  together  by  a  force  of  about  three-fourths  of  a  ton.     Fig.  89 
shows  the  manner  in  which  such  an  experiment  may  be  tried. 

Fig'90'  What  principle         579-  THE  HAND-GLASS.—  Fig. 

does  the  Hand-    90  is  nothing  more  than  a  tuui- 
glass  illustrate? 


the  top  and  bottom  ground  smooth,  so  as  to  fit 
the  brass  plate  of  the  air-pump.  Placing  it 
upon  the  plate,  cover  it  closely  with  the  palm 
of  the  hand,  and  work  the  pump.  Tbo  a;j 


PNEUMATICS.  15U 

within  the  glass  being  thus  exhausted,  the  hand  will  be  pressed 
down  by  the  weight  of  the  air  above  it :  on  readmitting  the  air, 
the  hand  may  be  easily  removed. 

What  principle  .580'  p™  BLADDER-GLASS.- 
is  illustrated  by  Fig.  91  is  a  bell-shaped  glass, 
the  ^Bladder-  COVered  with  a  piece  of  blad-. 
der,  which  is  tied  tightly  around 
its  neck.  Thus  prepared,  it  may  be  screwed 
to  the  plate  of  the  air-pump,  or  connected  with 
it  by  means  of  an  elastic  tube.  On  exhausting 
the  air  from  the  glass,  the  weight  of  the  external  air  on  the 
bladder  will  burst  it  inwards,  with  a  loud  explosion. 

***  92'  What  does  the        581.  THE  INDJA-RUBBER  GLASS. 

India-rubber  —  Fig.  92  is  a  glass  similar  to 
the  one  represented  in  the  last 
figure,  covered  with  india-rubber.  The  same 
experiments  may  be  made  with  this  as  were 
mentioned  in  tke  last  article,  but  with  different  results.  Instead 
of  bursting,  the  india-rubber  will  be  pressed  inwards  the  whole 
depth  of  the  glass. 

What    is    illus-  ^'    ^HE     FOUNTAIN-GLASS     AND    JET. Fig. 

trated  by  means  93  represents  the  jet,  which  is  a  small  brass 
of  the  Fountain-  tllbc  pi  94  ig  the  fountain-glass.  The  ex- 
elass  and  Jet ! 

periment  with  these  instruments  is  designed  to 

Kg.  93.     ghow  the   pressure  of  the  atmosphere   on      rie-  94- 
the  surface  of  liquids.     Screw  the  straight 
jet  to  the  stop-cock,  the  stop-cock  to  the 
fountain-glass,  with  the  straight  jet  inside 
of  the  fountain-glass,  and  the  lower  end  of 
the  stop-cock  to  the  plate  of  the  air-pump, 
and  then  open  the  stop-cock.     Having  ex- 
nausted  the  air  from  the  fountain-glass,  close  the  stop- 
sock,  remove  the  glass  from  the  pump,  and,  immersing 
it   in   a   vessel   of  water,   open  the  stop-cock.     The   pressure 
of  the  air  on  the  surface  of  the  water  will  cause  it  to  rush  up 
into  the  glas?  like  a  fountain. 


160  NATURAL   PHILOSOPHY. 

How  are  the        583.  PNEUMATIC  SCALES  FDR  WEIGHING  AIR.— 

Pneumatic         Fig.  95  represents  the  flask,  rig.  95. 

Scales  used?      QJ.  glagg  yeggel  and  gcaleg  for 

vveighing  air.  Weigh  the  flask  when  full 
of  air  ;  then  exhaust  the  air  and  weigh  the 
tiusk  again.  The  difference  between  its 
present  and  former  weight  is  the  weight  of 
the  air  that  was  contained  in  the  flask. 

What  princi-  .  584'  THE  SUCKER.  —  A 
pie  does  "  the  cL-cular  piece  of  wet  leather,  with  a  string 
Sucker  "illus-  attached  to  the  centre,  being  pressed  upon  a 
smooth  surface,  will  adhere  with  considerable 
tenacity,  when  drawn  upwards  by  the  string.  The  string  in 
this  case  must  be  attached  to  the  leather,  so  that  no  air  can  pass 
under  the  leather. 


What  is  the  **^*  -^HE  MERCURIAL  OR  WATER  TUBE.  —  : 
object  of  the  Exhaust  the  air  from  a  glass  tube  three  feet 

long  fitted  With  a  st°P-cock  at  one  end'  and  then 
immerse  it  in  a  vessel  containing  mercury  or 

water.  On  turning  the  stop-cock,  the  mercury  will  rise  to  the 
height  of  nearly  thirty  inches  ;  or,  if  immersed  in  water,  the 
water  will  rise  and  fill  the  tube,  and  would  fill  it  were  it  thirty 
feet  long.  This  experiment  shows  the  manner  in  which  water 
is  raised  to  the  boxes  or  valves  in  common  water-pumps. 

How  is  the  elas-  *^6.  EXPERIMENTS  SHOWING  THE  ELASTICITY 
ticity  of  the  air  OF  THE  AIR.  —  Place  an  india-rubber  bag,  or  a 
illustrated?  bladder,  partly  inflated,  and  tightly  closed,  un- 

der the  receiver,  and,  on  exhausting  the  air,  the  air  within  the 
bag  or  bladder,  expanding,  will  fill  the  bag.  On  readmitting 
the  air,  the  bag  will  collapse.  The  experiment  may  also  be 
made  with  some  kinds  of  shrivelled  fruit,  if  the  skin  be  sound. 
The  internal  air,  expanding,  will  give  the  fruit  a  fresh  and  plump 
appearance,  which  will  disappear  on  the  readmission  of  the  air 

587.  The  same  principle  may  be  illustrated  by  the  india- 


PNEUMATICS.  lt>i 

ruober  and  bladder  glasses,  if  they  have  stop-cocks  tc   confine 
the  air. 

588.  A  small  bladder  partly  filled  with  air  may  be  sunk  in  a 
vessel  of  water  by  means  of  a  weight,  and  placed  under  the 
receiver.  On  exhausting  the  air  from  the  receiver,  the  air  in 
the  bladder  will  expand,  and,  its  specific  gravity  being  thua 
diminished,  the  bladder  with  the  weight  will  rise.  On  read- 
mitting the  air,  the  bladder  will  sink  again. 

TT  589.    AlR  CONTAINED  IN  WATER  AND  IN  WOOD. 

now    can    trie 

presence  of  air    —  Place  a  vessel  of  water  under  the  receiver,  and, 

in  wood  be  de-    on  exhausting  the  air  from  the  receiver,  ihe  air 
tected?  .  .,...,,         .„        , 

in  the  water,  previously  invisible,  will  make  its 

appearance  in  the  form  of  bubbles,  presenting  the   semblance 
of  ebullition. 

590.  A  piece  of  light  porous  wood  being  immersed  in  the 
water  below  the  surface,  the  air  will  be  seen  issuing  in  bubble* 
from  tho  pores  of  the  wood. 

Explain  che  prin-        59L  THE  PNEUMATIC  BALLOON.—     "*»« 
ciple  of  the  Pneu-    Fig.  96  represents  a  small  glass  bal- 
matic  Balloon.         loonj  witt  itg  car  immersed  in  a  jar 

of  water,  and  placed  under  a  receiver.  On  exhaust- 
ing the  air,  the  air  within  the  balloon,  expanding,  gives 
it  buoyancy,  and  it  will  rise  in  the  jar.  On  readmit- 
ting the  air,  the  balloon  will  sink. 

692.  The  experiment  may  be  performed  without  the 
air-pump  by  covering  the  jar  with  some  elastic  sub- 
stance, as  india-rubber.     By  pressing  on  the  elastic       Jm, 
covering  with  the  finger,  the  air  will  be  condensed,  the 
water  will  rise  in  the  balloon,  and  it  will  sink.     On  removing 
the  pressure,  the  air  in  the  balloon,  expanding,  will  expel  part 
of  the  water,  and  the  balloon  will  rise.    This  is  the  more  conve- 
nient mode  of  performing  the  experiment,  as  it  can  be  repeated 
at  pleasure  without  resort  to  the  pump. 

593.  The  following  is  a  full  explanation  :  —  The  pressure  on 
the  top  of  the  vessel  first  condenses  the  air  between  the  covei 


162  NATURAL   PHILOSOPHY. 

and  the  surface  of  the  water;  this  condensation  presses  upo& 
the  water  below,  and,  as  this  pressure  affects  every  portion  of 
the  water  throughout  its  whole  extent,  the  water,  by  its  upward 
pressure,  compresses  the  air  within  the  balloon,  and  makes  room 
for  the  ascent  of  more  water  into  the  balloon,  so  as  to  alter  the 
specific  gravity  of  the  balloon,  and  cause  it  to  sink.  As  soon 
as  the  pressure  ceases,  the  elasticity  of  the  air  in  the  balloon 
drives  out  the  lately-entered  water,  and,  restoring  the  former 
lightness  to  the  balloon,  causes  it  to  rise.  If,  in  the  commence- 
ment of  this  experiment,  the  balloon  be  made  to  nave  a  specific 
gravity  too  near  that  of  water,  it  will  not  rise  of  itself, 
after  once  reaching  the  bottom,  because  the  pressure  of  the 
water  then  above  it  will  perpetuate  the  condensation  of  the  air 
which  caused  it  to  descend.  It  may  even  then,  however,  be 
made  to  rise,  if  the  perpendicular  height  of  the  water  above  it 
be  diminished  by  inclining  the  vessel  to  one  side. 

594.  This  experiment  proves  many  things  ;  namely  : 

First.  The  materiality  of  air,  by  the  pressure  of  the  hand!  on  the 
top  being  communicated  to  the  water  below  through  the  air  in  the 
upper  part  of  the  vessel. 

Secondly.  The  compressibility  of  air,  by  what  happens  in  the 
globe  before  it  descends. 

Thirdly.  The  elasticity,  or  elastic  force  of  air,  when  the  water  is 
expelled  from  the  globe,  on  removing  the  pressure. 

Fourthly.    The  lightness  of  air,  in  the  buoyancy  of  the  globe. 

Fifthly.  It  shows  that  the  pressure  of  a  liquid  is  exerted  in  all  direC' 
tions,  because  the  effects  happen  in  whatever  position  the  jar  be 
held. 

Sixthly.  It  shows  that  pressure  z>  as  the  depth,  because  less  press- 
ure of  the  hand  is  required  the  further  the  globe  has  descended  in 
the  water. 

Seventhly.  It  exemplifies  many  circumstances  of  fluid  support 
A  person,  therefore,  who  is  familiar  with  this  experiment,  and  can 
explain  it,  has  learned  the  principal  truths  of  Hydrostatics  and 
Pneumatics. 

595.  The  Pneumatic  Balloon  also  exhibits  the  principle  on  which 
the  well-known  glass  toy,  called  the  Cartesian  Devil,  is  constructed  ; 
and   it  may  be  thus  explained:    Several  images  of  glass,  hollow 
within,  and  each  having  a  small  opening  at  the  heel  by  which  water 
may  pass  in  and  out,  may  be  made  to  manoeuvre  in  a  vessel  of 
water.     Place  them  in  a  vessel  in  the  same  manner  with  the  bal- 
loon. Out,  by  allowing  different  quantities  of  water   to   enter   thf 


PNEUMATICS.  1(>3 

Apertures  in  the.  images,  cause  them  to  differ  a  little  from  one 
another  in  specific  gravity.  Then,  when  a  pressure  is  exerted  on 
the  cover,  the  heaviest  will  descend  first,  and  the  others  follow  in 
the  order  of  their  specific  gravity  ;  and  they  will  stop  or  return  to 
the  surface  in  reverse  order,  when  the  pressure  ceases.  A  person 
exhibiting  these  figures  to  spectators  who  do  not  understand  them, 
while  appearing  carelessly  to  rest  his  hand  on  the  cover  of  the  ves- 
sel, seems  to  have  the  power  of  ordering  their  movements  by  his 
will.  If  the  vessel  containing  the  figures  be  inverted,  and  the  cover 
be  placed  over  a  hole  in  the  table,  through  which,  unobserved,  press- 
ure can  be  made  by  a  rod  rising  through  the  hole,  and  obeying  the 
foot  of  the  exhibitor,  the  most  surprising  evolutions  may  be  pro- 
duced among  the  figures,  in  perfect  obedience  to  the  word  of  com- 
mand. 

596.  EXPERIMENTS   WITH    CONDENSED  AIR.— 
What    ts    the     m         ^  T-. 

use  of  the  Con-     -"-HE    CONDENSING   AND  EXHAUSTING  SYRINGE. — 

densing  and  The  Condensing  Syringe  is  the  air-pump  reversed. 
The*  Exhausting  Syringe  is  the  simple  air-pump 
without  its  plate  or  stand.  These  implements 
are  used  respectively  with  such  parts  pig.  97. 

of  the  apparatus  as  cannot  conveniently 
be  attached  to  the  a;.r-puinp,  and  as 
an  addition  to  such  pumps  as  do  not 
perform  the  double  office  of  exhaustion 
and  condensation.  In  some  sets  of 
apparatus  the  condensing  and  exhaust- 
ing syringes  are  united,  and  are  made 
io  perform  each  office  respectively,  by 
merely  reversing  the  part  which  con- 
tains the  valve. 

For  what  purpose         597-      THE      AlR' 

is  the  Air-cham-    CHAMBER. —  The  air- 

krused?  chamber,  Fig.  97,  is 

a  hollow  brass  globe  prepared  for  the  reception  of  a  stop-cock, 

and  is  designed  for  the  reception  of  condensed  air.   It  is  made 

in  different  forms  in  different  sets,  and  is  used  by  screwing  it  to 

a  condensing  pump  or  a  condensing  syringe. 

What  prin-  **98.    STRAIGHT    AND    REVOLVING    JETS    FROM 

tiple  <*f  Pneu-    CONDENSED    AIR.  —  Fill   the   air-chamber   (Fig. 

7* 


164  NATURAL   PHILOSOPHY. 

.    .„       97)  partly  with  water,   and  then  eondense  the 
mattes  is  illus-        '    "       J 
trated  by  the      air.     Then  confine  the  air  by  turning  the  cock  ; 

straight  and      after  which,  unscrew  it  from  the  air-pump,  and 
revolving  jets?   ^QW  ^  ^  Btra;ght  Qr  ^  revolving  jet     Then 

open  the  stop-cock,  and  the  water  will  be  thrown  from  the 
chamber  in  the  one  case  in 
a  straight  continued  stream, 
in  the  other  in  the  form  of 


a  wheel.     Figs.  98  and  99 
represent    a    view    of   the 

straight  and  the  revolving  jets.  In  the  revolving  jet 
the  water  is  thrown  from  two  small  apertures  made  at 
each  end  on  opposite  sides,  to  assist  the  revolution.  The 
circular  motion  is  caused  by  the  reaction  of  the  water  on  the 
opposite  sides  of  the  arms  of  the  jets  ;  for,  as  the  water  is  forced 
into  the  tubes,  it  exerts  an  equal  pressure  on  all  sides  of  the 
tubes,  and,  as  the  pressure  is  relieved  on  one  side  by  the  jet- 
hole,  the  arm  is  caused  to  revolve  in  a  contrary  direction.  This 
experiment,  performed  with  the  straight  jet,  illustrates  the 
principle  on  which  "Hero's  ball"  and  "Hero's  fountain"  are 
constructed.  % 

Explain  the  ^9.   THE  PRINCIPLE  OF  THE  AIR-GUN. — With 

principle  of       the  air-chamber,  as  in  the  last  experiments,  a 
the  Air-gun.       gmall  bragg  cylinder  or  gim.barrei5  Fig.  100,  may 

be  substituted  for  the  jets,  and  loaded  with  a  small  shot  fig.  10o 
or  paper  ball.  On  turning  the  cock  quickly,  the*  con- 
densed air,  rushing  out,  will  throw  the  shot  to  a  consider- 
able distance.  In  this  way  the  air-gun  operates,  an 
apparatus  resembling  the  lock  of  a  gun  being  substituted 
for  the  stop-cock,  by  which  a  small  portion  only  of  the 
condensed  air  is  admitted  to  escape  at  a  time ;  so  that 
the  chamber,  being  once  tilled,  will  afford  two  or  three  dozer* 
discharges.  The  force  of  the  air-gun  has  never  been  eqial  to 
more  than  a  fifteenth  of  the  force  of  a  common  charge  of  powder, 
and.  the  loudness  of  the  report  made  in  its  discharge  is  always 
as  great  in  proportion  to  its  force  as  that  of  the  comuou  gun 


PNEUMATICS.  1W. 

600.  Condensed  air  may  be  weighed  in  the 
iir  'tv/tat  must  air-chamber,  but,  in  estimating  its  weight,  the 
\lways  he  temperature  of  the  room  must  always  be  taken 
into  consideration,  as  the  density  of  air  is  ma- 
terially affected  by  heat  and  cold. 

What  doc  th        ^^'    ^XPERIMENTS   SHOWING  THE  INERTIA  OF 
Guinea  and      AIR.  —  THE  GUINEA  AND  FEATHER  DROP.  —  The 

Feather  Drop    inertia  of  air  is  shown  by  the  guinea  and  feather 
illustrate?  ,.,.A.  J .  .  ,.  ,      ,, 

drop,    exhibiting   the   resistance  which    the   air 

opposes  to  falling  bodies.  This  apparatus  is  made  in  different 
forms,  some  having  shelves  on  which  the  Kg.  102. 
guinea  and  feather  rest,  and,  when  ihe  air  is 
exhausted,  they  are  made  to  fall  by  the  turn- 
ing of  a  Jiandle.  A  better  form  is  that  repre- 
sented in  Fig.  101,  in  which  the  guinea  and 
feather  (or  a  piece  of  brass  substituted  for  the 
guinea)  are  enclosed,  and  the  apparatus  being 
screwed  to  the  plate  of  the  pump,  the  air  is 
exhausted,  a  stop-cock  turned  to  prevent  the 
readmission  of  the  air,  and  the  apparatus  being 
then  unscrewed,  the  experiment  may  be  repeatedly 
shown  by  one  exhaustion  of  the  air.  It  will  then 
appear  that  every  time  the  apparatus  is  inverted  the 
guinea  and  the  feather  will  fall  simultaneously.  The 
two  forms  of  the  guinea  and  feather  drop  are  ex- 
hibited in  Figs.  101  and  102,  one  of  which,  Fig.  101,  is  fur- 
nished with  a  stop-cock,^  the  other,  Fig.  102,  with  shelves. 

What  prin-  602.    EXPERIMENTS   SHOWING    THE    FLUIDITY   OP 

ciple  is  explain-  AIR.  —  THE  WEIGHT-LIFTER. — The  upward  press- 
erf  by  means  of  „  ^        .  ,,  ^,  A.        »•*'«.». 
the  weight-         ure  °* ™Q  air'  one  °* tne  Pr°Perties  of  its  fluidity, 
liftsr?               may  be  exhibited  by  an  apparatus   called   the 

*  Most  sets  of  philosophical  apparatus  are  furnished  with  stop-cocks 
and  elastic  tubes,  for  the  purpose  of  connecting  the  several  parts  with  the 
pump,  or  with  one  another.  In  selecting  the  apparatus,  it  is  important 
to  have  the  screws  of  the  stop-cocks  and  of  all  the  apparatus  of  similar 
thread,  in  order  that  every  article  may  subserve  as  many  purposes  as  pos- 
sible This  precaution  is  suggested  by  economy,  as  well  as  by  co  iveuieuc* 


166  NATURAL    PHILC  SOPHY. 

weight-lifter,  made  in  different  forms,  but  all 
on  the  same  principle.  The  one  represented 
in  Fig.  103  consists  of  a  glass  tube,  of  large 
bore,  set  in  a  strong  case  or  stand,  sup- 
ported by  three  legs.  A  piston  is  accu- 
rately fitted  to  the  bore  of  the  tube,  and  a 
hook  is  attached  to  the  bottom  of  the  piston, 
from  which  weights  are  to  be  suspended. 
One  end  of  the  elastic  tube  is  to  be  screwed 
to  the  plate  of  the  pump,  and  the  other 
end  attached  to  the  top  of  this  instrument. 
The  air  being  then  exhausted  from  the  tube,  the  weights  will  ba 
raised  the  whole  length  of  the  glass.  The  number  of  pounds' 
weight  that  can  be  raised  by  this  instrument  may  be  estimated 
by  multiplying  the  number  of  square  inches  in  the  bottom  of 
the  piston  by  fifteen. 

Explain  the  603.  THE  PNEUMATIC  SHOWER-BATH.  —  On  the 

Pneumatic  principle  of  the  upward  pressure  of  the  air  the 
pneumatic  shower-bath  is  constructed.  It  con- 
sists of  a  tin  vessel  perforated  with  holes  in  the  bottom  for  the 
shower,  and  having  an  aperture  at  the  top,  which  is  opened  or 
closed  at  pleasure  by  means  of  a  spring-valve.  [Instead  of  the 
spring-valve,  a  bent  tube  may  be  brought  round  from  the  top 
down  the  side  of  the  vessel,  with  an  aperture  in  the  tube  below 
the  bottom  of  the  vessel,  which  may  be  covered  with  the  thumb.] 
On  immersing  the  vessel  thus  constructed  in  a  pail  of  water, 
with  the  valve  open,  and  the  tube  (if  it  have  one)  on  the  outside 
of  the  pail,  the  water  will  fill  the  vessel.  The  aperture  then 
being  closed  with  the  spring  or  with  the  thumb,  and  the  vessel 
being  lifted  out  of  the  water,  the  upward  pressure  of  the  an 
will  confine  the  water  in  the  vessel.  On  removing  the  thumb 
or  opening  the  valve,  the  water  will  descend  in  a  shower,  untiJ 
the  vessel  is  emptied. 

What  tw  ^^'  -M-ISCELLANEOU8   EXPERIMENTS  DEPENDING 

properties         ON  TWO  on  MOKE  OF  THE  PROPERTIES  OF  AIR.— 


PNEUMATICS.  167 

of  air  are          THE  BOLT-HEAD   AND   JAR.  —  Fig.  104,  a   glasb 

illustrated  ly    globe  with  a  long  neck,  called  a  bolt-head  (or 
means  of  the  ,,     „,,    ,     .,, 

Bolt-head  and  anJ  long-necked  bottle),  partly  tilled  with  water, 


is  inverted  in  a  jar  of  water  (colored  with  a  few 
drops  of  red  ink  or  any  coloring  matter,  in  order      Fig  104 
that  the  effects  may  be  more  distinctly  visible),  and 
placed  under  the  receiver.     On  exhausting  the  air  in 
the  receiver,  the  air  in  the  upper  part  of  the  bolt- 
head,  expanding,  expels  the  water,  showing  the  elas- 
ticity of  the  air.     On  readmitting  the  air  to  the 
receiver,  as  it  cannot  return  into  the  bolt-head,  the 
pressure  on  the  surface  of  the  water  in  -the  jar  forces 
the  water  into  the  bolt-head,  showing  the  pressure 
of  the  air  caused  by  its  weight.     The  experiment 
may  be  repeated  with  the   bolt-head  without   any 
water,  and,  on  the  readmission  of  the  air,  the  water  will  nearly 
fill  the  bolt-head,  affording  an  accurate  test  of  the  degree  of 
exhaustion. 

What  two  605.    ^HE    TRANSFER    OF  _  FLUIDS    FROM    ONH 

principles  are  VESSEL  TO  ANOTHER.  —  The  experiment  may  bo 

concerned  in  made  with  two  bottleg  tightly  closed.     Let  one 

the  transfer  of  6      J 

fluids  from  De  partly  tilled  with  water,   and   the  two  con- 

one  vessel  to  nected  by  a  bent  tube,  connecting  the  interior  of 
the  empty  bottle  with  the  water  of  the  other,  and 
extending  nearly  to  the  bottom  of  the  water.  On  exhausting 
the  air  from  the  empty  bottle,  the  water  will  pass  to  the  other, 
and,  on  readmitting  the  air,  the  water  will  return  to  its  original 
position,  so  long  as  the  lower  end  of  the  bent  tube  is  below  the 
surface. 

What  ex  eri-  ®®®m  EXPERIMENTS  WITH  THE  SIPHON.  —  Close 
msnts  are  per-  the  shorter  end  of  the  siphon  with  the  finger  or 
formed  with  with  a  stop-cock,  and  pour  mercury  or  water  into 
the  longer  side.  The  air  contained  in  the  shorter 
side  will  prevent  the  liquid  from  rising  in  the  shorter  side. 
But,  if  the  shorter  end  be  opened,  so  as  to  afford  free  passage 


lt>&  NATURAL   PHILOSOPHY. 

outwards  for  the  air,  the  fluid  will  rise  to  an  equilibrium  m 
both  arms  of  the  siphon. 

607.  Pour  any  liquid  into  the  longer  arm  of  the  siphon  until 
*ne  shorter  arm  is  filled.  Then  close  the  shorter  end,  tc  pre- 
vent the  admission  of  the  air ;  the  siphon  may  then  be  turned 
in  any  direction  arid  the  fluid  will  not  run  out,  on  account  of 
the  pressure  of  the  atmosphere  against  it.  But,  if  the  shorter 
end  be  unstopped,  the  fluid  will  run  out  freely. 

What  effect  is  608>  AlR  ESSENTIAL  T0  ANIMAL  LIFE.  —  If 
produced  on  an  . 

animal  placed     an  animal  be  placed  under  the  receiver,  and  the 

under  an  ex-  air  exhausted,  it  will  immediately  droop,  and,  if 
cewer?  ™~  the  air  be  not  sPeei%  readmitted,  it  will  die. 

609.  AIR   ESSENTIAL  TO  COMBUSTION. —  Place 
How  is  it 
shown  that  air    &  lighted  taper,  cigar;  or  any  other  substance  that 

is  essential  to     wiu  produce  smoke,  under  the  receiver,  and  ex*- 
haust  the  air ;  the  light  will  be  extinguished,  and 
the  smoke  will  fall,  instead  of  rising.     If  the  air  be  readmitted, 
the  smoke  will  ascend. 

What  effect  is        610.    THE    PRESSURE   OP    THE   AIR    RETARDS 

^herundlran  EBULLITION.*  —  Ether,  alcohol,  and  other  distilled 

exhausted  re-  liquors,  or  warm  water,  placed  under  the  receiver, 

caver  ?  wj}}  appear  to  boil  when  the  air  is  exhausted. 

„„      a.     ,  611.  The  existence  of  many  bodies  in  a  liquid 

the  pressure  of  f°rm  depends  on  the  weight  or  pressure  of  the 
the  air  on  the  atmosphere  upon  them.  The  same  force,  like- 
lodies  *  wise,  prevents  the  gases  which  exist  in  fluid  and 

solid  bodies  from  disengaging  themselves.  If,  by 
rarefying  the  air,  the  pressure  on  these  bodies  be  diminished, 
they  either  assume  the  form  of  vapors,  or  else  the  gas  detaches 
itself  altogether  from  the  other  body.  The  following  experi- 
ment proves  this :  Place  a  quantity  of  lukewarm  water,  milk 
or  alcohol,  under  a  receiver,  and  exhaust  the  air,  and  the  liquid 

*  EBULLITION.  —  The  operation  of  boiling.  The  agitation  of  liquor  bj 
teat,  which  throws  it  up  iuto  bubbles. 


PNEUMATICS.  169 

will  either  pass  off  in  vapor,  or  will  have  the  appearance  of 

boiling. 

612.  An  experiment  to  prove  that  the  pressure 
What  expert-  r  . 

ment  shows        of  the  atmosphere  preserves  some  bodies  in  the 

that  the  liquid    liquid  form  may  thus  be  performed.     Fill  a  long 

form  of  some       .  -.  ,   v       ,       •,  T       .,, 

bodies  is  de-       v     '  or  a  closed  at  one  end,  with  water,  and 

pendent  on  invert  it  in  a  vessel  of  water.  The  atmospheric 
atmospheric  pressure  wm  retain  the  water  in  the  vial.  Then, 
pressure  *.  5  „  i  •  • ""  i 

by  means  or  a  bent  tube,  introduce  a  lew  drops 

of  sulphuric  ether,  which,  by  reason  of  their  small  specific 
gravity,  will  ascend  to  the  top  of  the  vial,  expelling  an  equal 
bulk  of  water.  Place  the  whole  under  the  receiver,  and  ex- 
naust  the  air,  and  the  ether  will  be  seen  to  assume  the  gaseous 
form,  expanding  in  proportion  to  the  rarefaction  of  the  air 
under  the  receiver,  so  that  it  gradually  expels  the  water  from 
the  vial,  and  fills  lip  the  entire  space  itself.  On  readmitting 
the  a«r,  the  ether  becomes  condensed,  and  the  water  will  re- 
ascend  into  the  vial. 

a  613.  A  simple  and  interesting  experiment  con 

water  be  frozen  nected  with  the  science  of  chemistry  may  thus  b^ 
under  a  rt-  performed  by  means  of  the  air-pump.  A  watch- 
glass,  containing  water,  is  placed  over  a  small 
vessel  containing  sulphuric  acid,  and  put  under  the  bulbed 
receiver.  When  the  air  is  exhausted,  vapor  will  freely  rise 
from  the  water,  and  be  quickly  absorbed  by  the  acid.  An 
intense  degree  of  cold  is  thus  produced,  and  the  water  will 
freeze. 

614.  In  the  above  experiment,  if  ether  be  used  instead  of  the 
acid,  the  ether  will  evaporate  instead  of  the  water,  and,  in  the 
process  of  evaporation,  depriving  the  water  of  its  heat,  the 
water  will  freeze.  These  two  experiments,  apparently  similar 
ha  effects,  namely,  the  freezing  of  the  water,  depend  upon  two 
different  principles  which  pertain  to  the  science  of  chemistry. 

What  is  the  615>    THE  PNEUMATIC    PARADOX.  — An   inter- 

Pneumatic  .  ..«-•*       *\i 

Paradox?         esting  experiment,  illustrative  or  the  pueaiuatic 


170  NATURAL    PHILOSOPHY. 

paradox,  may  be  thus  performed :  Pass  a  small  open  tube  (yjs 
a  piece  of  quill)  through  the  centre  of  a  circular  card  two  or 
three  inches  in  diameter,  and  cement  it,  the  lower  end  passing 
down,  and  the  upper  just  even  with  the  card.  Then  pass  a  pin 
through  the  centre  of  another  similar  card,  and  place  it  on 
the  former,  with  tne  pin  projecting  into  the  tube  to  prevent 
the  upper  card  from  sliding  off.  It  will  then  be  impossible 
to  displace  the  upper  card  by  blowing  through  the  quill, 
on  account  of  the  adhesion  produced  by  the  current  passing 
between  the  discs.  On  this  principle  smoky  chimneys  have 
ur^en  remedied,  and  the  office  of  ventilation  mpre  effectually 
performed. 

61G'  WlND'  —  Wind  i8  ^  Put  in  motion. 

617.  There  are  two  ways  .in  which  the  motion 
In  what  two  .  .  . 

ways  may  the  °*  tne  air  mav  anse-  «  maj  be  considered  as 
motion  of  the  an  absolute  motion  of  the  air,  rarefied  by  heat 
a/r  5T  an<^  condensed  by  cold ;  or  it  may  be  only  an 

apparent  motion,  caused  by  the  superior  velocity 
of  the  earth  in  its  daily  revolution. 

618.  When  any  portion  of  the  atmosphere  is  heated  it  becomes 
rarefied,  its  specific  gravity  is  diminished,  and  it  consequently 
rises.  The  adjacent  portions  immediately  rush  into  its  place,  to 
restore  the  equilibrium.  This  motion  produces  a  current  which 
pushes  into  the  rarefied  spot  from  all  directions.  This  is  what 
we  call  wind. 

619.  The  portions  north  of  the  rarefied  spot 
v^ndlav^ed?  Pro^uce  a  north  wind,  those  to  the  south  produce 
a  south  wind,  while  those  to  the  east  and  west 
in  like  manner,  form  currents  moving  in  opposite  directions 
At  the  rarefied  spot,  agitated  as  it  is  by  winds  from  all  direc- 
tions, turbulent  and  boisterous  weather,  whirlwinds,  hurricanes, 
rain,  thunder  and  lightning,  prevail.  This  kind  of  weather 
occurs  most  frequently  in  the  torrid  zone,  where  the  heat  is 
greatest.  The  air,  being  more  rarefied  there  than  in  any  other 


PNEUMATICS.  171 

part  jf  the  globe,  is  lighter,  and,  consequently,  ascends ;  that 
about  the  polar  regions  is  continually  flowing  from  the  poles  to 
the  equator,  to  restore  the  equilibrium;  while  the  air  rising 
from  the  equator  flows  in  an  upper  current  towards  the  poles, 
so  that  the  polar  regions  may  not  be  exhausted. 

620.  A  regular  east  wind  prevails  about  the 
What  wind 
prevails  in  the    equator,  caused  in  part  by  the  rarefaction  of  the 

equatorial          air  produced  by  the  sun  in  his  daily  course  from 
'  east  to  west.     This  wind,  combining  with  that 

xrom  the  poles,  causes  a  constant  north-east  wind  for  about  thirty 
degrees  north  of  the  equator,  and  a  south-east  wind  at  the 
same  distance  south  of  the  equator. 

621.  From  what  has  now  been  said,  it-  appears  that  there  is  a 
circulation  of  air  in  the  atmosphere  ;  the  air  in  the  lower  strata 
flowing  from  the  poles  to  the  equator,  and  in  the  upper  strata 
flowing  back  from  the  equator  to  the  poles.  It  may  here  be  re- 
marked, that  the  periodical  winds  are  more  regular  at  sea  than  on 
the  land  ;  and  the  reason  of  this  is,  that  the  land  reflects  into  the 
atmosphere  a  much  greater  quantity  of  the  sun's  rays  than  the 
water,  therefore  that  part  of  the  atmosphere  which  is  over  the  land 
is  more  heated  and  rarefied  than  that  which  is  over  the  sea.  This 
occasions  the  wind  to  set  in  upon  the  land,  as  we  find  it  regularly 
does  on  the  coast  of  Guinea  and  other  countries  in  the  torrid  zone. 
There  are  certain  winds,  called  trade-winds,  the  theory  of  which 
raav  be  easily  explained  on  the  principle  of  rarefaction,  affected,  as 
it  is,  by  the  relative  position  of  the  different  parts  of  the  earth  with 
the  sun  at  different  seasons  of  the  year,  and  at  various  parts  of  the 
day.  A  knowledge  of  the  laws  by  which  these  winds  are  controlled 
is  of  importance  to  the  mariner.  When  the  place  of  the  sun  with 
respect  to  the  different  positions  of  the  earth  at  the  different  seasons 
of  the  year  is  understood,  it  will  be  seen  that  they  all  depend  upon 
the  same  principle.  The  reason  that  the  wind  generally  subsides 
at  the  going  down  of  the  sun  is,  that  the  rarefaction  of  the  air,  in 
the  particular  spot  which  produces  the  wind,  diminishes  as  the  sun 
declines,  and,  consequently,  the  force  of  the  wind  abates.  The 
great  variety  of  winds  in  the  temperate  zone  is  thus  explained. 
The  air  is  an  exceedingly  elastic  fluid,  yielding  to  the  slightest 
pressure  ;  the  agitations  in  it,  therefore,  caused  by  the  regular 
winds,  whose  causes  have  been  explained,  must  extend  every  way 
to  a  great  distance,  and  the  air,  therefore,  in  all  climates  will  suffer, 
more  or  less  perturbation,  according  to  the  situation  of  the  country, 
the  position  of  mountains,  valleys,  and  a  variety  of  other  causes 
Hence  every  climate  must  be  liable  to  variable  winds.  The  (fuality 
of  winds  is  affected  by  the  countries  over  which  they  pass  ?  uud 


172 


NATURAL    PHILOSOPHY. 


ciiey  fire  sometimes  rendered  Destilential  by  the  heat  of  deserts  or 
the  imtrid  exhalations  of  marshes /and  lakes.  Thus,  from  the 
deserts  of  Africa,  Arabia  and  the  neighboring  countries,  a  hot  wind 
blows,  called  Samiel,  or  Simoon,  which  sometimes  produces  instant 
death.  A  similar  wind  blows  from  the  desert  of  Sahara,  upon  the 
western  coast  of  Africa,  called  the  Harmattan,  producing  a  dryness 
and  heat  which  is  almost  insupportable,  scorching  like  the  blasts 
of  a  furnace. 

622.    WHIRLWINDS   AND   WATERSPOUTS.  —  The 
How  is  wind 
sometimes  af-     direction  ot  winds  is  sometimes  influenced  by  the 

form  of  lofty  and  precipitous  mountains,  which, 
resisting   their    direct    course,   causes   them   to 
descend  with  a  spiral  and  whirling  motion,  and 
with  great  force. 

623.  A  similar  effect  is  produced  by  two  winds  meeting  at  an 
angle,  and  then  turning  upon  a  centre.  If  a  cloud  happen  to  be 
between  these  two  winds  thus  encountering  each  other,  it  will  be 
condensed  and  rapidly  turned  round,  and  all  light  substances  will 
be  carried  up  into  the  air  by  the  whirling  motion  thus  produced. 

What  is  sup-         624.  The  whirlwind,  occurring   at   sea,  occa« 


fected  by  the 
face  of  a 
country  ? 


to  *e  the 


cause  oj   vater- 
tpouts  ?  spout 


iong   ^     singular   phenomenon   of    the   water 


Fig.  3U6. 


ACOUSTICS.  173 

What  doe*  ^25.  Fie.  105  represents  the  several  forma  in 

Fig.  105  rep-       .  . 

resent  *  which  water-spouti  are  sometimes  seen. 

626.  From  a  dense  cloud  a  cone  descends  in  the  form  of  a  trumpet, 
with  the  small  end  downwards.     At  the  same  time,  the  surface  of 
the  sea  under  it  is  agitated  and  whirled  round,  the  waters  are  con- 
verted into  vapor,  and  ascend  with  a  spiral  motion,  till  they  unite 
with  the  cone  proceeding  froin  the  cloud.     Frequently,  however, 
they  disperse  before  the  junction  is  effected.     Both  columns  diminish 
towards  their  point  of  contact,  where  they  are  sometimes  not  more 
than  three  or  four  feet  in  diameter.     In  the  centre  of  the  water- 
spout there  is  generally  a  vacant  space,  in  which  none  of  the  small 
particles  of  water  ascend.     In  this,  as  well  as  around  the  outer 
edges  of  the  water-spout,  lurge  drops  of  rain  fall     Water-spouts 
sometimes   disperse    suddenly,    and   sometimes   continue   to   move 
rapidly  over  the  surface  of  the  sea,  continuing  sometimes  in  sight 
for  the  space  of  a  quarter  of  an   hour      When  the  wate^-spout 
breaks,  the  water  usually  descends  in  the  form  of  heavy  rain.     It  is 
proper  here  to  observe  that  by  some  authorities  the  phenomena  of 
water-spouts  are  considered  as  due  to  electrical  causes. 

627.  A  notion  has  prevailed  that  water-spouts  are  dangerous  to 
shipping.     It  is  true  that  small  vessels  incur  a  risk  of  being  overset 
if  they  carry  much  sail,  because  sudden  gusts  of  wind,  from  all 
points  of  the  compass,  are  very  common  in  the  vicinity  of  water- 
spouts;    but  large  vessels,  under   but  a  small  spread  of  canvas, 
encounter,  as  is  said,  but  little  danger. 

628.  Pneumatics  forms  a  branch  of  physical  science  which  has 
been  entirely  created  by  modern  discoveries.     Galileo  first  demon- 
strated that  air  possesses  weight.     His  pupil,  Torricelli,  invented 
the  barometer;  and  Pascal,  by  observing  the  difference  of  the  alti- 
tudes of  the  mercurial  column  at  the  top  and  the  foot  of  the  Puy  de 
Dome,  proved  that  the  suspension  of  the  mercury  is  caused  by  the 
pressure  -of  the  atmosphere.     Otto  Guericke,  a  citizen  of  Magde- 
burg, invented  the  air-pump  about  the  year  1654  ;  and  Boyle  und 
Manotte  soon  after  detected,  by  its  means,  the  principal  mechanical 
properties  of  atmospheric   air.     Analogous   properties  have  been 
proved  to  belong  to  all  the  other  aeriform  fluids.     The  problem  of 
determining  the  velocity  of  their  vibrations  was  solved  by  Newton 
and  Euler,  but  more  completely  by  Lagrange.     The  theoretical  prin- 
ciples relative  to  the  pressure  and  motion  of  elastic  fluids,  from 
which   the   practical   formulae   are   deduced,  were   established   by 
Daniel   Bernoulli   in   his   Hydrodynamica  (1738),  but   have   bee& 
rendered  more  general  by  Navier. 

What  is  629.  ACOUSTICS. — Acoustics  is  the -science 

Acoustics?    which  treats  of  the  nature  and  laws  of  sound 
It  includes  the  theory  of  musical  concord  or  harmony. 


'74  NATURAL   PHILOSOPHY. 

\Vhat  is  630.  Sound  is  the  sensation  produced  in  the 
*>mnd?  organs  Of  hearing  by  the  vibrations  or  undulations 
transmitted  through  the  air  around.* 

631.  If  a  bell  be  rung  under  an  exhausted  receiver,  no  sound  can 
be  heard  from  it ;  but  when  the  air  is  admitted  to  surround  the  bell, 
the  vibrations  immediately  produce  sound. 

032.  Again,  if  the  experiments  be  made  by  enclosing  the  bell  in 
a  small  receiver,  full  of  air,  and  placing  that  under  another  receiver, 
from  which  the  air  can  be  withdrawn,  though  the  bell,  when  struck, 
must  then  produce  sound,  as  usual,  yet  it  will  not  be  heard  if  the 
outer  receiver  be  well  exhausted,  and  care  be  taken  to  prevent  the 
vibrations  from  being  communicated  through  any  solid  part  of  the 
apparatus,  because  there  is  no  medium  through  which  the  vibrations 
of  the  bell  in  the  smaller  receiver  can  be  communicated  to  the  ear.f 

Why  is  a  sound  ^^'  Bounds  are  louder  when  the  air  sur- 
louder  in  cold  rounding  the  sonorous  body  is  dense  than  when 
weather?  ft  js  m  a  rarefied  state,  and  in  general  the 

intensity  of  sound  increases  with  the  density  of  the  medium 
by  which  it  is  propagated. 

634.  For  this  reason  the  sound  of  a  bell  is  louder  in  cold  than 
in  warm  weather ;  and  sound  of  any  kind  is  transmitted  to  a 
greater  distance  in  cold,  clear  weather,  than  in  a  warm,  sultry 
day.  On  the  top  of  mountains,  where  the  air  is  rare,  the  human 
voice  can  be  heard  only  at  the  distance  of  a  few  rods ;  and  the 
iring  of  a  gun  produces  a  sound  scarcely  louder  than  the  crack- 
ing of  a  whip. 

What  are  So-  635.  Sonorous  bodies  are  those  which  pro- 
norous  bodies  ?  ^uce  ciearj  distinct,  regular,  and  durable 
sounds,  such  as  a  bell,  a  drum,  wind  instruments,  musical 
strings  and  glasses.  These  vibrations  can  be  communicated 
to  a  distance  not  only  through  the  air,  but  also  through 
liquids  and  soM  bodies. 

*  "  The  sensation  of  sound  is  produced  by  the  wave  of  air  impinging  on 
the  membrane  of  the  ear-drum,  exactly  as  the  momentum  of  a  wave  of  the 
sea  would  strike  the  shore."  —  [ Lardner.] 

t  In  performing  these  experiments,  the  bell  must  be  placed  in  such  a  man- 
aer  thac  whatever  supports  it  will  rest  on  a  soft  cushion  of  wool,  so  as  to 
prevent  the  vibrations  from  being  communicated  to  the  plate  of  the  air 
putup,  or  any  other  of  tUe  solid  parts  of  the  apparatus. 


ACOUSTICS.  175 

To  what  do        636.  Bodies   owe   their  sonorous   property 

bodies  owe  their ,       ,,.        ,       ..,  -r,  ,,          ,.. 

sonorous  prop- io  thelr   elasticity.     But,  although  it   is  un- 

erties  ?  doubtedly  the  case  that  all  sonorous  bodies  are 

elastic,  it  is  not  to  be  inferred  that  all  elastic  bodies  are 
sonorous. 

637.  The  vibrations  of  a  sonorous  body  give  a  tremulous  or  un- 
dulatory  motion  to  the  air  or  the  medium  by  which  it  is  surrounded, 
similar  to  the  motion  communicated  to  smooth  water  when  a  stono 
is  thrown  into  it. 

What  are  the  638.  Sound  'l&  communicated  more  rapidly 
best  conductors,  and  with  greater  power  through  solid  bodies 
than  through  the  air,  or  fluids.  It  is  conducted 
by  water  about  four  times  quicker  than  by  air,  and  by  solids 
about  twice  as  rapidly  as  by  water. 

639.  If  a  person  lay  his  head  on  a  long  piece  of  timber,  he  can 
hear  the  scratch  of  a  pen  at  the  other  end,  wnile  it  could  not  be 
heard  through  the  air. 

640.  If  the  ear  be  placed  against  a  long,  dry   brick  wall,  and  a 
person  strike  it  once  with  a  hammer,  the  sound  will  be  heard  tivice, 
because  the  wall  will  convey  it  with  greater  rapidity  than  the  air, 
though  each  will  brin^  it  to  the  ear. 

641.  It  is  on  the  principle  of  the  greater  power  of  solid  bodies  ti 
communicate  sound  that  the  instrument  called  the  Stethoscope  *  is 
constructed. 

What  is  the       642.  The  Stethoscope  is  a  perforated  cylin- 

Stethoscope,     fer  Of  li^ht,  fine-drained  wood,  with  a  funnel- 

and  on  what  '  .        °    .       .  ; 

principle  is  it  shaped  extremity,  which  is  applied  externally  to 

constructed?    fa   cavities   of  the   body,  to   distinguish   the 
sounds  within. 

T YJ  t  •   ,7  643.  By  means  of  the  stethoscope  the  phy- 

of  the  stetho-        sioian  is  enabled  to  form  an  opinion  of  the  healthy 
scope  ?  action  of  the  lungs,  and  other  organs  to  which  the 

ear  cannot  be  directly  applied. 

*  The  word  Stethoscope  is  derived  from  two  Greek  words,  art &oS,  the 
breast,  and  <rxo7i:st»,  to  examine,  and  is  given  to  this  instrument  because  't 
is  applied  to  the  breast  of  a  person  for  the  purpose  of  ascertaining  the  oon- 
dition  of  the  lungs  and  other  internal  organs.  Dr.  Webster  suggests  that 
the  term  Pkonophorus,  or  Sound-conductor,  would  be  a  preferable  name  lor 
fclie  instrument. 


176  NATUBAL    PHILOSOPHY 

With  what  rapidity     644.   Sound    passing    through    the   air 
does  sound  move?   moveg  at  tne  rate  Of  ;Q20  feet  jn  a  secorKJ 

of  time ;  and  this  rule  applies  to  all  kinds  of  sound,  whether 
loud  or  soft.* 

What  kind  of          645.  The  softest  whisper,  tnerefore,  flies  as  fast 
sounds  move         as  the  loudest  thunder ;  and  the  force  and  direction 
of  the  wind,  although  they  affect  the  continuance 
of  a  sound,  have  but  slight  effect  on  its  velocity. 

646.  Were  it  not  for  this  uniform  velocity  of  all  kinds  of  sound, 
the  music  of  a  choir,  or  of  an  orchestra,  at  a  short  distance,  woulc* 
be  but  a  strange  confusion  of  discordant  sounds  ;   for  the  different 
instruments  or  voices,  having  different  degrees  of  loudness,  could  not 
simultaneously  reach  the  ear. 

647.  The  air  is  a  better  conductor  of  sound  when  it  is  humid  than 
when  it  is  dry.     A  bell  can  be  more  distinctly  hoard  just  before  a 
rain  ;  and  sound  is  heard  better  in  the  night  than  in  the  day,  because 
the  air  is  generally  more  damp  in  the  night. 

648.  The  distance  to  which  sound  may  be  heard  depends  upon 
various  circumstances,  on  which  no  definite  calculations  can  be  pre- 
dicated.    Volcanoes,  among  the  Andes,  in  South  America,  have  been 
heard  at  the  distance  of  three  hundred  miles  ;  naval  engagements 
have  been  heard  two   hundred;  and  even  the  watchword  "All  '5 
toe//,"  pronounced  by  the  unassisted  human  voice,  has  been  heard 
from  Old  to  New  Gibraltar,  a  distance  of  twelve  miles.     It  is  said 
that,  the  cannon  fired  at  the  battle  of  Waterloo  were  heard  at  Dover. 

649.  A  clear  and  frosty  atmosphere  is  favorable  to  the  trans- 
mission of  Sound,  especially  where  the  surface  over  which  it  passes  is 
smooth  and  level.     Conversation  in  the  polar  regions  has  been  carried 
on  between  persons  more  than  a  mile  apart.     The  cannon  in  naval 
engagements  in  the  English  Channel  have  been  heard  in  the  centre 
of  England. 

650.  A  blow  struck  under  the  water  of  the  Lake  of  Geneva  was 
heard  across  the  whole  breadth  of  the  lake,  a  distance  of  nine  miles. 
The   earth  itself  is  a  good   conductor  of  sound*    The  trampling  of 
horses  can  be  heard  at  a  great  distance  by  putting  the  ear  to  the 
ground,  and  the  approach  of  railroad-cars  can  be  ascertained  when 
very  far  off  by  applying  the  ear  to  the  rail. 

*  The  velocity  of  sound  has  sometimes  been  estimated  as  much  as  eleven 
nundred  and  forty -two  feet  in  a  second.  The  state  of  the  air  must,  h<  wever, 
be  taken  into  consideration.  The  higher  the  temperature,  the  greater  tha 
velocity;  and  it  has  been  ascertained  that  within  certain  limits  the  velocity 
is  increased  about  one  foot  for  every  degree  that  the  thermometer  rises.  Ex- 
periments made  with  a  cannon  at  midnight  by  Arago,  Gay  Lassac,  and 
others,  when  the  thermometer  stood  at  61°,  gave  1118.39  feet  per  second  as 
the  velocity  of  sound.  The  rate  stated  in  No.  644  will  not  therefore  be  far 
from  the  truth.  The  experiments  which  gave  a  result  >i  leven  hundred  and 
forty-two  feet  in  a  second  were  probably  made  when  the  weather  was  e*- 
trmufcl  warm 


ACOUSTICS.  177 

To  what  prao       651.  This  uniform  velocity  of  sound  enab  es  us 

velocit™  lofh"  to  ascertain»  with  some  Degree  of  accuracy,  the 
sound  applied?  distance  of  an  object  from  which  it  proceeds.     — 

If,  for  instance,  the  flash  of  a  gun  at  sea  is  seen  a  half  of  a  minute 
before  the  report  is  heard,  the  vessel  must  be  at  the  distance  of  about 
six  miles. 

652.  In  the  same  manner  the  distance  of  a  thunder-cloud  may 
be  estimated  by  counting  the  seconds  thi.t  intervene  between  the 
flash  of  the  lightning  and  the  roaring  of  the  thunder,  and  multiplying 
them  by  1120. 

™       .      7          653,  THE    ACOUSTIC    PARADOX.  —  Sound,    as    has 
i    is  me    a]reaciy  5een  stated,  is  propagated  by  the  undulations 
Acoustic  Vara- 


of  ^  ^      Now?  ag  these  undu]ation8  or  waves  are 

precisely  analogous  to  the  case  of  two  series  of  waves 
formed  upon  the  surface  of  a  liquid,  there  is  a  point  where  the 
elevation  of  a  wave,  produced  by  one  cause,  will  coincide  with  the 
depression  of  another  wave  produced  by  another  cause,  and  the  con- 
sequence will  be  neither  elevation  nor  depression  of  the  liquid. 


Explain  the  ®^'  ^ien'  therefore,  two  sounds  are  produced 
acoustic  para-  in  different  places,  there  is  a  point  between  them 
dox-  where  the  undulations  will  counteract  each  other, 

r.nd  the  two  sounds  may  produce  silence. 

655.  A  simple   illustration   of  this   fact   may  be   made  with   a 
tuning-fork.     If  this  instrument  be  put  into  vibration  and  held  up  tc 
the  ear  and  rapidly  turned,  the  sound,  instead  of  being  continuous, 
will  appear  to  be  pulsative  or  interrupted  ;  but,  if  slowly  caused  to 
revolve  at  a  distance  from  the  ear,  a  position  of  the  forks  will  be 
found  at  which  the  sound  will  be  inaudible. 

656.  A  similar  experiment  may  be  made  with  the  tuning-fork 
iield  over  a  cylindrical  glass  vessel.     Another  glass  vessel  of  similai 
kind  being  placed  with    its  mouth  at  right  angles  to  the  first,  no 
sound  will  be  heard  ;  but,  if  either  cylinder  be  removed,  the  sound 
will  be  distinctly  audible  in  the  other.     The  silence  produced  in  this 
way  is  due  to  the  interference  of  the  undulations. 

This  seeming  paradox,  when  thus  explained,  like  the  paradox 
mentioned  under  the  heads  of  Hydrostatics  and  Pneumatics,  and 
another  to  be  mentioned  under  the  head  of  Optics,  will  be  found 
to  be  perfectly  consistent  with  the  laws  of  sound. 
What  is  657.  An  echo  is  produced  by  the  vibrations  of  the 
an  echo?  air  meeting  a  hard  and  regular  surface,  such  as  a  wall, 
a  rock,  a  mountain,  and  being  reflected  back  to  the  ear,  thus 
producing  the  same  sound  a  second  and  sometimes  a  third  and 
fourth  time. 


178  NATUKAL  PHILOSOPHY. 

Why  are  there         ^58.  For  this  reason,  it  is  evident  tbat  no  echo 
no  echoes  «£  sea    can  be  heard  at  sea,  or  on  an  extensive  plain, 
where  there  are  no  objects  to  reflect  the  sound. 

Bywhatlawis  659.  Sound,  as  well  as  light  and  heat,  is  re- 
sound reflected  ?  fleeted  in  obedience  to  the  same  law  that  has 
already  been  stated  in  Mechanics— namely,  the  angles  of  inci- 
dence and  of  reflection  are  always  equal. 

660.  It  is  only  necessary,  therefore,  to  know  how  sound  strikes 
against  a  reflecting  surface  to  know  how  it  will  be  reflected.  It  is 
related  of  Dionysius,  the  tyrant  of  Sicily,  that  he  had  a  dungeon 
(called  the  ear  of  Dionysius)  in  which  the  roof  was  so  constructed 
as  to  collect  the  words  and  even  the  whispers  of  the  prisoners  con- 
fined therein,  and  direct  them  along  a  hidden  conductor  to  the  place 
where  he  sat  to  listen,  and  thus  he  became  acquainted  with  the 
most  secret  expressions  of  his  unhappy  victims. 

What  is  said         661.    Speaking-trumpets  do  not  depend  for 

of  speaking- 

trumpets?  .        their  efficiency  upon  the  reflection  of  sound. 

662.  The  voice,  instead  of  being  diffused  in  the  open  air,  is  con- 
fined within  the  trumpet,  and  the  vibrations  imparted  by  the  lips 
to  the  column  of  air  within  the  trumpet  produce  better  waves  in 
the  open  air  than  the  lips  alone  would  be  able  to  do.  Speaking- 
trumpets  are  chiefly  used  by  naval  officers  to  aid  the  voice,  so  that 
the  word  of  command  may  be  heard  above  the  sound  of  winds  and 
waves. 

How  is  a  hear-  663-  Hearing-trumpets,  or  the  trumpets  used 
ing-trumpet  by  deaf  persons,  are  also  constructed  on  the  same 
e  '  principle ;  but  as  the  voice  enters  the  large  end 
of  the  trumpet  instead  of  the  small  one,  it  is  not  so  much  con- 
fined, nor  so  much  increased.* 

664.  The  musical  instrument  called  the  trumpet  acts  also  on  the 
same  principle  with  the  speaking-trumpet,  so  far  as  its  form,  tends 
to  increase  the  sound. 

665.  The  smooth  and  polished  surface  of  the  interior  parts  of 
certain  kinds  of  shells,  particularly  if  they  be  spiral  or  undulating, 

*  In  this  connection  the  author  cannot  refrain  from  giving  publicity  to 
the  value  of  a  pair  of  acoustic  instruments  worn  by  one  of  the  members  of 
his  family.  They  consist  of  two  small  hearing-trumpets  of  a  peculiar  con- 
struction, connected  by  a  slender  spring  with  an  adjusting  slide,  which, 
passing  over  the  head,  keeps  both  trumpets  in  their  place.  They  are  con- 
cealed from  observation  by  the  head-dress?  and  enable  the  wearer  to  join 
in  conversation  of  ordlc&ry  tone,  from  which  without  them  she  is  wholly 
debarred. 


ACOUSTICS.  |7'«' 

dt  them  to  collect  ar4  reflect  the  various  sounds  which  are  taking 
place  in  the  vicinity.  Hence  the  Cyprias,  the  Nautilus,  and  som-o 
other  shells,  when  held  near  the  ear,  give  a  continued  sound,  whici 
resembles  the  roar  of  the  distant  ocean. 

On  what  prin-  666.  Sound,  like  light,  after  it  has  been  reflect 
wAis  erin  *•-  e(^  ^rom  severa^  surfaces  may  be  collected  into  one 
galleries  cot,-  point,  as  a  focus,  where  it  will  be  more  audibk 
<tructed?  tnaa  jn  any  other  part;  and  on  this  principle 

whispering-galleries  may  be  constructed. 

667.  The  famous  whispering-gallery  in    the  dome  of  St.  Paul's 
church,  in  London,  is  constructed  on   this  principle.     Persons   at 
rery  remote  parts  of  the  building  can  carry  on  a  conversation  in  a 
soft  whisper,  which  will  be  distinctly  audible  to  one  another,  while 
others  in  the  building  cannot  hear  it  ;  and  the  ticking  of  a  watch 
may  be  heard  from  side  to  side. 

668.  There   is  a  church  in  the  town  of  Newburyport,  in  Massa 
chusetts,  which,  as  was  accidentally  discovered,  has  the  same  prop- 
erty as  a  whispering-gallery.     Persons  in  opposite  corners  of  the 
building,  by  facing  the  wall,  may  carry  on  a  conversation  in  the 
softest  whisper,  unnoticed  by  others  in  any  other  part  of  the  build 
ing.     It  is  the  building  which  contains  in  its  cemetery  the  remain* 
of  the  distinguished  preacher,  Wh  icefield. 

What  is  an  669.  ACOUSTIC  TUBES.  —  Sounds  may  be  coi: 
AcousticTube  ?  veyed  to  a  much  greater  distance  through  contin- 
uous tubes  than  through  the  open  air.  The  tubes  used  to  con- 
rey  sounds  are  called  Acoustic  Tubes.  They  are  much  used  iu 
public  houses,  stores,  counting-rooms,  &c.,  to  convey  communi- 
cations from  one  room  to  another. 

670.  The  quality  of  sound  is  affected  by  the  furniture  of  a  room, 
particularly  the  softer  kinds,  such  as  cur  tains,  carpets,  &c.;  because 
having  little  elasticity,  they  present  surfaces  unfavorable  to  vibni 
tions. 

671.  For  this  reason,  music  always  sounds  better  in  rooms  with 
bare  walls,  without  carpets,  and  without  curtains.     For  the  sanv 
reason,  a  crowded  audience  increases  the  difficulty  of  speaking. 

672.  As  a  general  rule,  it  may  be  stated  that  plane  and  smooth 
tur  faces  reflect  sound  without  dispersing  it;  convex  surfaces  disperse  it. 
ind  concave  surfaces  collect'  it. 


How    is    the         ^^'  ~^e  soun(^   °^  tne  aumaa   voice    is    pro 
round  of  the    duced  by  the  vibration  of  two  delicate  membrane? 

human   voice    situated  at  the  top  of  the  windpipe,  between  whicb 
produced?  .  f 

the  air  from  the  lungs  passes. 

8 


180  NATURAL    PHILOSOPHY.  * 

674.  The  tones  are  varied  from  grave  to  acute,  by  penmg  01 
contracting  the  passage  ;  and.  they  are  regulated  by  the  muscles 
belonging  to  the  throat,  by  the  tongue,  and  by  the  cheeks.  The 
management  of  the  voice  depends  much  upon  cultivation  ;  and 
although  many  persons  can  both  speak  and  sing  with  ease,  and  with 
great  power,  without  much  attention  to  its  culture,  yet  it  is  found 
that  they  who  cultivate  their  voices  by  use  acquire  a  degree  of  flexi- 
bility and  ease  in  its  management,  which,  in  a  great  measure,  guj» 
plies  the  deficiency  of  nature.* 

675.  Ventriloquism  t  is  the  arc  of  speaking  ir 

•  ,     l-     ?       such  a  manner  as  to  cause  the  voice  to  appear 

to  proceed  from  a  distance. 

676.  The  art  of  ventriloquism  was  not  unknown  to  the  ancients  , 
and  it  is  supposed  by  some  author^  that  the  famous  responses  of  the 
oracles  at  Delphi,  at  Ephesus,  <KC.,  were  delivered  by  persons  who 
possessed  this  faculty.  There  is  no  doubt  that  many  apparently 
wonderful  pieces  of  deception,  which,  in  the  days  of  superstition 
and  ignorance,  were  considered  as  little  short  of  miracles,  were  per- 
formed by  means  of  ventriloquism.  Thus  houses  have  been  made 

*  Dr.  Rush's  very  valuable  work   on  "  The  Philosophy  of  the  Human 
Voice  "  contains  much  valuable  matter  in  relation  to  the  human  voice 
Dr.  Barber's  "  Grammar  of  Elocution,"  and  the  "  Rhetorical  Reader,"  by 
the  author  of  this  volume,  contain  useful  instructions  in  a  practical  form. 
To  the  work  of  Dr.  Rush  both  of  the  latter  works  are  largely  indebted. 

f  The  word  Ventriloquism  literally  means,  "  speaking  from  the  belly,"  and 
It  is  so  defined  in  Chambers'  Dictionary  of  Arts  and  Sciences.  The  ven- 
triloquist, by  a  singular  management  of  the  voice,  seems  to  have  it  in  his 
power  "  to  throw  his  voice  "  in  any  direction,  so  that  the  sound  shall  appear 
to  proceed  from  that  spot.  The  words  are  pronounced  by  the  organs  usu- 
ally employed  for  that  purpose,  but  in  such  a  manner  as  to  give  little  or  no 
motion  to  the  lips,  the  organs  chiefly  concerned  being  those  of  the  throat 
and  tongue.  The  variety  of  sounds  which  the  human  voice  is  capablu  of 
thus  producing  is  altogether  beyond  common  belief,  and,  indeed,  is  truly 
surprising.  Adepts  in  this  art  will  mimic  the  voices  of  all  ages  and  condi- 
tions of  human  life,  from  the  smallest  infant  to  the  tremulous  voice  of  tot- 
tering age,  and  from  the  intoxicated  foreign  beggar  to  the  high-bred,  arti- 
ficial tones  of  the  fashionable  lady.  Some  will  also  imitate  the  warbling 
of  the  nightingale,  the  loud  tones  of  the  whip-poor-will,  and  the  scream  of 
the  peacock,  with  equal  truth  and  facility.  Nor  are  these  arts  confined  to 
professed  imitators  ;  for  in  many  villages  boys  may  be  found  who  are  in 
f.he  habit  of  imitating  the  brawling  and  spitting  of  cats  in  such  a  manner 
as  to  deceive  almost  every  hearer. 

The  human  voice  is  also  capable  of  imitating  almost  every  inanimate 
sound.  Thus,  the  turning  and  occasional  creaking  of  a  grindstone,  with 
the  rush  of  the  water,  the  sawing  of  wood,  the  trundling  and  creaking 
of  a  wheeltirrow,  the  drawing  of  corks,  and  the  gurgling  of  the  flow- 
ing liquor,  the  sound  of  air  rushing  through  a  crevice  on  a  wintry  night 
oad  a  great  variety  of  other  noises  of  the  same  kind,  are  imitated  by  the 
voice  so  exactly  as  to  deceive  any  hearer  who  does  not  know  whence  tbej 
proceed 


ACOUSTICS.  181 

t<>  appear  haunted,  voices  have  been  heard  from  tomls,  and  the  iead 
have  been  made  to  appear  to  speak,  to  the  great  dismay  of  the 
neighborhood,  by  means  of  this  wonderful  art. 

Ventriloquism  is,  without  doubt,  in  great  measure  the  gift  of 
nature  ;  but  many  persons  can,  with  a  little  practice,  utter  sounds 
and  pronounce  words  without  opening  the  lips  or  moving  the  mus- 
cles of  the  face  ;  and  this  appears  to  be  the  great  secret  of  the 
art. 

How  is  the  ®^'  ^USICAL  SOUNDS,  OR  HARMONY.  —  The 
soundofamu-  sound  produced  by  a  musical  string  is  caused  by 
steal  ^string  its  vibrations  ;  and  the  height  or  depth  of  the 
tone  depends  upon  the  rapidity  of  these  vibra- 
tions. Long  strings  vibrate  with  less  rapidity  than  short  ones; 
and  for  this  reason  the  low  tones  in  a  musical  instrument  pro- 
ceed* from  the  long  strings,  and  the  high  tones  from  the  short 
ones.  That  character  of  sound  depending  upon  rapidity  of  vi- 
bration is  called  pitch. 

678.  Fig.  106.    AB  represents  a  musical  string. 
Explain      Tf  ±f  r,p 
Fig.  106.     "  tt  b  Fig-  m 

drawn  __„  ___  G--  ...... 

up  to  G-,  its  elas-  ^-'"'.--  -----  ^  '""---IT"^^ 

ticity  will  not  on-          x^  '^*-  -  _  ~ 


ly   carry  it  back  *x**>~7  --------  &  _________  l 

again,     but    will  *"**-*..  F  -------  ~,-'''* 

*~-~  —  iy  ____  --~ 

give  it  a  momen- 

tum which  will  carry  it  to  H,  from  whence  it  will  successively 
return  to  T,  F,  C,  D,  &c.,  until  the  resistance  of  the  air  entirely 
destroys  its  motion. 

0      ,  679.    The  pitch  ^of  the   sound  produced  by 

the  pitch  of  the     strings  depends  upon  their  length,  thickness, 

tone  of  a  string  weight,  and  degree  of  tension.  The  pitch  oi 
depend  f 

the  sound  produced  by  wind  instruments  de- 

pends upon  their  size,  their  length,  and  their  internal  diameter. 

680.  When  music  is  made  by  the  use  of  strings,  the  air  is  struck 
by  the  body,  and  the  sound  is  caused  by  the  vibrations  ;  when  it  is 
made  by  pipes,  the  body  is  struck  by  the  air  ;  but  as  action  and  re- 
action are  equal,  the  effect  is  the  same  in  both  cases. 

681.  Long  and  large   strings,  when  loose,  produce  the  lowest 


NATURAL    PHILOSOPHY 

tones  but  differet-t  tones  may  be  produced  from  the  saii.e  string, 
according  to  -the  degree  of  tension.  Large  wind  instruments,  also, 
produce  the  lowest  tones ;  but  different  tones  may  be  produced 
from  the  same  instrument,  according  to  the  distance  of  the  aperture 
for  the  escape  of  the  wind  from  the  aperture  where  it  enters. 

How  does  the         682-  Tb.e  qu  dity  of  the  sound  of  all  musical 

temperature  of     , 

the  weather  of-    instruments  is  anected  by  the  changes  in  the 

feet  the  tone  of    temperature  and  specific  gravity  of  the  atmos- 
a   musical   in-       , 
ttrument?  Pnere' 

683.  As  heat  expands  and  cold  contracts  the  materials  of  which 
the  instrument  is  made,  it  follows  that  the  strings  will  have  a 
greater  degree  of  tension,  and  that  pipes  and  other  wind  instru- 
ments will  be  contracted,  or  shortened,  in  cold  weather.  For  this 
reason,  most  musical  instruments  are  higher  in  tone  (or  sharper) 
in  cold  weather,  and  lower  in  tone  (or  more  flat)  in  warm  weather 

On  what  is  the  ^^'  ^ne  science  of  harmony  is  founded  on 
science  of  har-  the  relation  which  the  vibrations  of  sonorous 
many  founded?  bodieg  have  to  each  other> 

685.  Thus,  when  the  vibrations  of  one  string  are  double  those  ol 
another,  the  chord  of  an  octave  is  produced.     If  the  vibrations  of 
two  strings  be  as  two  to  three,  the  chord  of  a  fifth  is  produced. 
When  the  vibrations  of  two  strings  frequently  coincide,  they  pro- 
duce a  musical  chord  ;  and  when  the  coincidence  of  the  vibrations 
is  unfrequent,  discord  is  produced. 

686.  A  simple  instrument,  called  a  monochord,  contrived  for  the 
purpose  of  showing  the  length  and  degree  of  tension  of  a  string  tft 
produce  the  various  musical  tones,  and  to  show  their  relations,  may 
thus  be  made.     A  single  string  of  catgut  or  wire,  attached  at  one 
end  to  a  fixed  point,  is  carried  over  a  pulley,  and  a  weight  is  sus- 
pended to  the  other  end  of  the  string.     The  string  rests  on  two 
bridges,  between  the  fixed  point  and  the  pulley,  one  of  which  is 
fixed,  the  other  movable.     A  scale  is  placed  beneath  the  string  by 
which  the  length  of  the  vibrating  part  between  the  two  bridges 
may  be  measured.    By  means  of  this  simple  instrument,  the  respect- 
ive lengths  required  to  produce  the  seven  successive  notes  of  the 
gamut  will  be  as  follows  :  it  being  premised  that  the  longer  the 
gtring  the  slower  will  be  its  vibrations. 

637.  Let  the  length  of  the  string  required  to  produce  the  note 
called  C  be  1  ;  the  length  of  the  string  to  produce  the  successive 
aotes  will  be 

CDEFGA         B         C 

J    I    t    J    I    «    A -4- 


ACOUSTICS. 


183 


688.  Hence,  the  octave  will  require 
only  lialf  of  the  length  of  the  fundamen- 
tal 'note,  and  the  vibrations  that  produce 
it  will  be  as  two  to  one.  The  vibrations 
of  the  string  in  producing  the  successive 
notes  of  the  scale  will  be  as  follows  : 


C        D       E        F        O        A 

1    *    t    I    i"f 


That  is,  to  produce  the  note  D  nine  vibra- 
tions will  be  made  in  the  same  time  that 
eight  are  made  by  C,  five  of  E  to  four  of 
0,  four  of  F  to  three  of  C,  three  of  G 
to  two  of  C,  five  of  A  to  three  of  C, 
fifteen  of  B  to  eight  of  C,  and  two  of 
the  octave  C  to  one  ol  the  fundamen- 
tal C. 

689.  The  same  relations  exist  in  each 
successive  octave  throughout  the  whole 

nusical  scale. 

690.  As    harmony   depends   upon  the 
coincidence  of  vibrations,  it  follows  that 
the  octave  produces  the  most  perfect  har- 
mony ;    next   in    order   is    the   fifth,    as 
every  third  vibration  of  the  fifth  corre- 
sponds with  every  second  vibration  of  the 
fundamental.     Next  to  the   fifth  in  the 
order  of  harmony  follows  the  fourth,  and 
after  the  fourth  the  third. 

691.  The   following   scale,    containing 
three   octaves,    exhibits  the   proportions 
which  exist  between  the  fundamental  and 
all  the  other  notes  within  that  compass. 

692.  In  the  lowest  line  of  this  scale 
the   numbers    show   the  intervals.     The 
figures    above   express   the    number    of 

'ibrations  of  the  fundamental  or  tonic, 
and  the  upper  line  of  figures  represents 
the  proportionate  vibrations  of  each  suc- 
cessive interval. 

693.  It  is  found  in  practice  that  when 
two    sounds    are    caused    by  vibrations 
which  are  in  some  simple  numerical  pro- 
portion to  each  other,  such  as   1  to  2,  or 
<2  to  3,  or  3  to  4,  &c.,  they  are  pleasing 
to  the  ear  ;  and  the  whole  science  of  har- 
oiony  is  founded  on  such  relations. 

694    The  principal  harmonies  are  the 
Kituvc,    fifth,   fourth,    major   third,    and 


-    ! 


|      ORB 


~f1 


- 


iffl 


INI  «s 
lljTj  - 
ilWIi  «s 


184  NATURAL   PHILOSOPHY. 

minor  third  ;  and  the  relations  between  them  and   the  fundamental 
or  tunic  are  as  follows  : 

Octave,  2  to  1. 

Fifth,  3   "  2. 

Fourth.  4   "  3. 

Major  Third,  5   "  4. 

Minor  Third,  6    "  5. 

695.  Tht  following  Rules  may  serve  as  the  basts  of  interesting 
calculations. 

(1.)  Strings  of  the  same  diameter  and  equal  tension  vibrate  in 
times  in  an  inverse  proportion  to  their  lengths. 

(2.)  The  vibrations  of  strings  of  equal  length  and  tension  are  in 
an  inverse  proportion  to  their  diameters. 

(3.)  The  vibrations  of  strings  of  the  same  length  and  diameter 
are  as  the  square  roots  of  the  weights  causing  their  tension. 

(4.)  The  vibrations  of  cylindric  tubes  closed  at  one  end  are  in  an 
inverse  proportion  to  their  length. 

(5.)  The  sound  of  tubes  open  at  both  ends  is  the  same  with  that 
of  tubes  of  half  the  length  closed  at  one  end. 

[The  limits  of  this  work  will  not  admit  the  further  consideration 
of  the  subject  of  Harmony.  It  constitutes  of  itself  a  science,  in- 
volving principles  which  require  deep  study  and  investigation  ;  and 
they  who  would  pursue  it  advantageously  will  scarcely  expect,  in 
the  pages  of  an  elementary  work  of  this  kind,  that  their  wants  will 
be  supplied.] 

696.      Questions  for  Solution. 

(1.)  A  rocket  was  seen  to  explode,  and  in  two  seconds  the  sound  of  the 
explosion  was  heard  ;  how  far  off  was  the  rocket  1  Ann.  2240 ft. 

(2.)  The  flash  from  a  cloud  was  seen,  and  in  five  seconds  the  thunder  was 
heard  ;  what  was  the  distance  of  the  cloud  1  Arts.  5600./Z. 

(3.)  A  musical  string  four  feet  long  gave  a  certain  tone;  what  must  be 
the  length  of  a  string  of  similar  size  and  tension  to  produce  the  note  of  a 
fifth  7  Ans.  2ft.  8  in. 

(4.)  A  certain  string  vibrates  100  times  in  a  second  ;  how  many  times 
must  a  string  of  the  same  kind  vibrate  to  produce  the  octave  1  the  fifth  1 
the  minor  third  1  the  major  third  1  Am.  200;  150;  120;  125. 

(5.)  Supposing  that  two  sounds,  with  all  their  attending  circumstances 
similar,  reach  an  ear  situated  at  the  point  of  interference  of  the  undula- 
tions, —  what  will  be  the  consequence  1  [See  Nos.  653  and  054.] 

(6.)  The  length  of  a  string  being  36,  what  will  be  length  of  its  octave  ! 
fifth  'J  fourth  1  major  and  minor  thirds  ]  Ans.  18;  24;  27;  28.8;  30. 

(7.)  A  stone,  being  dropped  into  a  pit,  is  heard  to  strike  the  bottom  in 
7  seconds  ;  how  deep  was  the  pit  1  Ans.  By  Algebra,  600,/i. 

[N.  B.  In  estimating  the  velocity  of  sound,  it  is  generally  reckoned  in 
practice  as  only  at  1090  feet  per  second,  supposing  the  thermometer  at  the 
freezing  point ;  and  one  foot  per  second  is  added  for  every  degree  of  tem 
perature  above  the  freezing  point,  or  32°.  The  average  rate  of  1120  fetyj 
Kaa  been  assumed  in  the  text.] 


PYRONOMICS.  185 

(8.)  Suppose  the  length  of  a  music-string  to  be  five  feet ;  what  will  bo 
the  length  of  the  15th,  all  other  circumstances  being  equal  ?  Ans.  4  in. 

(9.)  The  length  of  the  fifth  being  four,  what  will  be  the  length  of  the 
fundamental,  or  tonic  ?  Ans.  6. 

(10.)  What  must  be  the  length  of  a  pipe  of  an  open  diapason  to  produce 
the  same  tone  with  four  foot  C  of  the  stopped  diapason  ?  Ans.  ^ft. 

[N.  B.  The  open  diapason  consists  of  pipes  open  at  both  ends ;  the 
stopped  diapason  has  its  pipes  closed  at  one  end.  [See  No.  695  (5).] 

(11.)  In  what  proportion  are  the  vibrations  of  two  strings  of  equal 
length  and  diameter — one  stretched  with  a  weight  of  twenty-five  pounds, 
the  other  with  a  weight  of  fifty  pounds  ?  [See  No.  695  (3).]  Ans.  1  to  1.41  + 

(12.)  In  what  proportion  are  the  vibrations  of  two  strings  of  equal 
length  and  tension,  but  having  diameters  in  the  proportion  of  3  to  5  ? 

Ans.  5  to  3. 

What  is  697.     PYRONOMICS,     OK    THE     LAWS     OF 

Pyronomicsf  J£EAT. —  Pyronomics  is  the  science  which 
treats  of  the  laws,  the  properties  and  operations  of  heat. 

What  is  698.  Heat  is  now  known  to  be  a  motion  of  the 
Heat?  minutest  particles  (or  molecules)  of  a  body.  The 
molecules  of  all  known  bodies  are  continually  in  motion.  This 
motion  may  be  transmitted  from  one  body  to  another. 

What  is  699.  Cold  is  therefore  only  an  absence  or  partial 
Cold  ?  absence  of  this  motion  of  the  molecules.  We  say  a 
body  is  cold  when  the  motion  of  its  molecules  is  less  than  usual, 
or  less  than  that  of  surrounding  bodies. 

What  effect  has  700.  When  a  body  is  heated  to  a  high  tem- 
heat  on  bodies  ?  perature,  the  motion  of  the  molecules  becomes 
greater  and  greater,  and  the  whole  body  becomes  larger.  Heat 
and  the  attraction  of  cohesion  constantly  act  in  opposition  to 
each  other ;  hefice,  the  more  a  body  is  heated,  the  more  its  par- 
ticles will  be  separated.  (See  par.  1463.) 

701.  Heat  causes  most  substances  to  dilate  or  expand,  while 
cold  (which  is  merely  the  absence  of  heat)  causes  them  to  contract.* 
Since  there  is  a  continual  change  in  the  temperature  of  all  bodies 
on  the  surface  of  the  earth,  it  necessarily  follows  that  there  will  be 
a  constant  corresponding  change  in  their  magnitude  as  they  are 
affected  by  heat  and  cold.  They  expand  their  bulk  in  a  warm  day, 
and  contract  it  in  a  cold  one.  In  warm  weather  the  flesh  -swells, 

*  Two  exceptions  to  this  remark  are  water  and  clay.  Water  expand* 
when  it  freezes  ;  clay  contracts  when  heated. 


1  86  NJ»  TURAL    PHILOSOPHY. 

{he  blocl  vessels  are  well  filled,  the  hands  arid  the  feet,  as  will  an 
othei  parts  of  the  body,  expand  or  acquire  a  degree  of  plumpness 
,md  the  skin  is  distended  ;  while,  on  the  contrary,  in  cold  weathei 
the  flesh  appears  to  contract,  the  vessels  shrink,  and  the  skin 
Appears  shrivelled.  Hence  a  glove  or  a  shoe  which  is  too  tight  in 
the  summer  will  often  be  found  to  be  easy  in  cold  weather. 

702.  The  effect  of  heat  in  separating  the  particles  of  different 
Kinds  of  substances  is  seen  in  the  melting  of  solids,  such  as  metals, 
wax,  butter,  &o.  The  heat  insinuates  itself  between  the  particles, 
;md  forces  them  asunder.  These  particieo  th°n  are  removed  from 
that  degree  of  proximity  to  each  other  within  which  coxitsive  attrac 
ii(»n  exists,  and  the  body  is  reduced  to  a  fluid  form.  When  the 
heat  is  removed  the  bodies  return  to  their  former  solid  state. 


147.  t  k'  d  f  ^^'  -^eat  Passes  through  some  bodies  with 
'•at  lies  arrest  diore  difficulty  than  through  others,  but  there  is 

'he  progress  no  kind  of  matter  which  can  completely  arrest  its 
v,f  hf.at  ? 

progress.      (See  par.  1465.) 

What  is  704.  Of  all  the  effects  of  heat,  that  produced  upou 
fteam?  ,oter  is,  perhaps,  the  most  familiar.  The  particles 
are  totally  separated,  and  converted  into  steam  or  vapor,  and 
{heir  extension  is  wonderfully  increased.  The  steam  wlrch 
arises  from  boiling  water  is  nothing  more  than  portions  of  the 
water  heated.  The  heat  insinuates  itself  between  the  par- 
ticles of  the  water,  and  forces  them  asunder.  When  deprived 
of  the  heat,  the  particles  will  unite  in  the  form  of  drops  of 
•Arater. 

This  fact  can  be  seen  by  holding  a  cold  plate  over  boiling  water. 
The  steam  rising  from  t  ho  water  will  be  condensed  into  drops  on 
the  bottom  of  the  plate.  The  air  which  we  breathe  generally  con 
tains  a  considerable  portion  of  moisture.  On  a  cold  day  this 
moisture  condenses  on  the  glass  in  the  windows,  and  becomes 
visible.  We  see  it  also  collected  into  drops  on  the  outside  of  a 
tumbler  or  other  vessel  containing  cold  water  in  warm  weathor. 
Heat  also  produces  most  remarkable  effects  upon  air,  causing  it  to 
expand  to  a  wonderful  extent,  while  the  absence  of  heat  causes  it 
r.'j  "shrink  or  contract  into  very  small  dimensions. 

705.    The  attraction   of  cohesion   causes  the 

How  is  ™'M'        small  watery  particles  which   compose   mist  or 

vapor  to  unite  together  in  the  form  of  drops  of 

water.     It  is  thus  that  rain  is  produced.     The  clouds  consist  of 


PYKO^OMICS.  187 

mist  or  vapor  expanded  by  heat.  They  rise  to  the  cold  regions 
of  the  skies,  where  the  particles  of  vapor  lose  their  heat,  and 
then,  uniting  in  drops,  fall  to  the  earth.  But  so  long  as  they 
retain  their  heat  the  attraction  of  cohesion  can  have  no  influence 
upon  them  and  they  will  continue  to  exist  in  the  form  of  steam, 
vapor  or  mist. 

706.  The  thermometer,  an  instrument  designed  to  measure  degrees 
of  heat,  has  already  been  described,,  in  connexion  with  the  barom- 
eter, under  the  head  of  Pneumatics.     Heat,  under  the  name  of 
caloric,  is  properly  a  subject  of  consideration  in   the  science  of 
Chemistry.     It  exists  in  two  states,  called,  respectively,  free  heat 
and  latent  heat.     Free  heat,  or  free  caloric,  is  that  which  is  per- 
ceptible to  the  senses,  as  the  heat  of  a  fire,  the  heat  of  the  sun,  &c. 
Latent  heat  is  that  which  exists  in  most  kinds  of  substances,  but  is 
not  perceptible  to  the  senses  until  it  is  brought  out  by  mechanical 
or  chemical  action.     Thus,  when  a  piece  of  cold  iron  is  hammered 
upon  an  anvil,  it  becomes  intensely  heated  ;  and   when  a  small 
portion  of  sulphuric  acid,  or  vitriol,  is  poured  into  a  vial  of  cold 
water,  the  vial  and  the  liquid  immediately  become  hot.     A  further 
illustration  of  the  existence  of  latent  or  concealed  heat  is  given  at 
the  fireside  every  day.     A  portion  of  cold  fuel  is  placed  upon  the 
grate  or  hearth,  and  a  spark  is  applied  to  kindle  the  fire  which 
warms  us.     It  is  evident  that  the  heat  given  out  by  the  fuel,  when 
ignited,  does  not  all  proceed  from  the  spark,  noi  can  we  perceive  it 
in  the  fuel  ;  it  must,  therefore,  have  existed  somewhere  in  a  latent 
state.     It  is,  however,  the  effects  of  free  heat,  or  free  caloric,  which 
are  embraced  in  the  science  of  Pyronomics.     The  subject  of  latent 
heat  belongs  more  properly  to  the  science  of  Chemistry.    (See  par. 
1470.) 

707.  The  terms  heat  and  cold,  as  they  are  generally  used,  are 
merely  relative  terms  ;  for  a  substance  which  in  one  person  would 
excite  the  sensation  of  heat  might,  at  the  same  time,  seem  cold  to 
another.     Thus,  also,  to  the  same  individual  the  same  thing  may  be 
made  to  appear,  relatively,  both  warm  and  cold.     If,  for  instance,  a 
person  were  to  hold  one  hand  near  to  a  warm  fire,  and  the  other  on 
a  cold  stone,  or  marble  slab,  and  then  plunge  both  into  a  basin  of 
lukewarm  water,  the  liquid  would  appear  cold  to  the  warm  hand 
and  warm  to  the  cold  one. 


What  are  the  '    SOURCES  OF  HEAT.  —  The  four  prin- 

itrindpal          cipal  sources  of  the  development  of  heat  are 
twras  of         the  gunj  Electricity,  Chemical  Action  and  Me- 
chanical Action.     The  heat  produced  by  fire 
rvul  flame  is  due  to  chemical  action. 


IH8  .         NATURAL    PHILCSuPHY. 

I*T._,  •    ,L  709.  But,  of  all  the  sources  from  which  heat 

V\  hat  is  the 

•ource  of  the  has  been  developed  by  human  agency,  that  pro- 
greatest  degree  duced  by  electrical  action,  and  especially  the 
galvanic  battery,  is  by  far  the  most  eminent  in 
its  degree  and  in  its  effects.  It  can  reduce  the  most  refractory 
substances  to  a  fluid  state,  or  convert  them  to  their  original 
elements. 

710.  The   heat  generally  ascri  jeu   to  the   sun   is   attended   by 
peculiar  phenomena,  but  imperfectly  understood.     It  may,  perhaps. 
De  questioned  whether  there  be  any  absolute  heat  in  the  rays  of 
that  luminary,  for  we  find  that  the  heat  is  not  in  all  cases  propor- 
tionate  to  his  proximity.     Thus,  on  the  tops  of  high  mountains, 
and  at  great  elevation,  it  is  not  found  that  the  heat  is  increased, 
but,  on  the  contrary,  diminished.     But  there  are  other  phenomena 
which  lead  to  the  conclusion  that  his  rays  are  accompanied  by  the 
development  of  heat,  if  they  are  not  the  cause  and  the  source  of  it. 

711.  All  mechanical  operations  are  attended  by  heat.     Friction, 
sudden  compression,  violent  extension,  are  all  attended  by  heat. 
The  savage  makes  his  fire  by  the  friction  of  two  pieces  of  dry  wood. 
Air,  suddenly  and  violently  compressed,  ignites  dry  substances ;  * 
and  India-rubber  especially,  whan  suddenly  extended,  shows  evident 
signs  of  heat ;  and  an  iron  bar  may  be  made  red  hot  by  beating  it 
quickly  on  an  anvil.     Even  water,  when  strongly  compressed,  gives 
out  heat. 

What  are  the  712'     The    PrinciPal    effects    of    heat     arft 

principal  ef-      three,  namely  : 
f-cts  of  heat  i         ^  Heat  expan(is  most  substances  - 

(2.)  It  converts  them  from  a  solid  to  a  liquid  state. 

(3.)  It  converts  them  from  the  liquid  to  the  gaseous  state. 

713.  There  are  many  substances  on  which  ordinary  degiees  of 
heat,  and,  indeed,  heat  of  great  intensity,  seems  to  produce  no 
sensible  effects  ;  and  they  have,  therefore,  received  the  name  of 
incombustible  bodies.  Bodies  usually  called  incombustible  are 
generally  mineral  substances,  such  as  stones,  the  earths,  &c.  All 
vegetable  substances,  and  most  animal  substances,  are  highly  com- 
bustible. The  metals  also  all  yield  to  the  electrical  or  galvanic 
battery.  But  there  is  sufficient  evidence  that  all  bodies  were  once 
in  a  fluid  or  gaseous  state,  and  that  the  solid  forms  that  they  have 
assumed  are  due  to  the  loss  of  heat  Could  the  same  degree  of 

*  Syringes  have  been  constructed  on  this  principle  A  solid  piston 
being  forcibly  driven  downward  or  dry  tinder,  ignites  it 


PYKONOMICS.  189 

>  a  tensity  of  heat  be  restored,  it  is  presumed  that  they  would  resume 
tktiir  liquid  or  gaseous  form. 

What  is  the       714    Heat  tends  to  diffuse  itself  equally  through 

fast  law  of     „      .  ' 

heat  ?  a^  substances. 

If  a  heated  body  be  placed  near  a  cold  one,  the  temperature  of 
the  former  will  be  lowered,  while  that  of  the  latter  will  be  raised. 
All  substances  contain  a  certain  quantity  of  heat  ;  but,  on  account 
of  its  tendency  to  diffuse  itself  equally,  and  the  difference  in  the 
power  of  different  substances  to  conduct  it,  bodies  of  the  same 
absolute  temperature  appear  to  possess  different  degrees  of  heat. 

Thus,  if  the  hand  be  successively  applied  to  a  woollen  garment,  a 
mahogany  table,  and  a  marble  slab,  all  of  which  have  been  for  some 
time  in  the  same  room,  the  woollen  garment  will  appear  the  warmest, 
and  the  marble  slab  the  coldest,  of  the  three  articles  ;  but,  if  a  ther- 
mometer be  applied  to  each,  no  difference  in  the  temperature  will 
be  observed. 

What  is  the  ^'^'  -^rom  tnis  ^  appears  that  some  substances 

reason  that         conduct  heat  readily,  and  others  with  great  dif- 

wme  sub-  faculty.     The  reason  that  the  marble  slab  seems 

stances  feel         yi  ,     . 

warm  and          tae   coldest   is,  that  marble,   being  a   good   con- 

others  cold  in  ductor  of  heat,  receives  the  heat  from  the  hand 
thesameroom?  &Q  readily  that  the  loss  is  instantly  felt  by  the 
hand  ;  while  the  woollen  garment,  being  a  bad  conductor  of 
heat,  receives  the  heat  from  the  hand  so  slowly  that  the  los^  is 
imperceptible. 

What  is  the  716.  The    different    power    of    receiving    and 

difference  in  conducting  heat,  possessed  by  different  substances, 
the  warmth  of  is  the  cause  of  the  difference  in  the  warmth  of 

different  gar-     various  substances  used  for  clothing. 
ments  ? 


Why  are  1^-1  •  ^^us,   woollen   garments    are    warm    gar 

woollen  gar-    ments,   because    they  part  slowly  with  the    heat 

[G}i  they  ac(luire  from  the   body>   and'  cons°- 
quently,  they  do  not  readily  convey  the  warmth 

of  the  body  to  the  air  ;  while,  on  the  contrary,  a  linen  garment 
is  a  cool  one,  because  it  parts  with  its  heat  readily,  and  as  read- 
ily receives  fresh  heat  from  the  body.  It  is,  therefore,  con- 
>iantly  receiving  heat  fron-  the  body  arid  throw:ng  it  out  into 


190  NATURAL   PHILOSOPHY. 

the  air,  while  the  woollen  garment  retains  the  heat  which  it  re- 
ceives, and  thus  encases  the  body  with  a  warm  covering. 

718.  For  a  similar  reason,  ice  in  summer  is  wrapped  in  woollen 
cloths.  It  is  then  protected  from  the  heat  of  the  air,  and  will  not 
melt. 

How  is  Jieat  719.  Heat  is  propagated  in  two  ways  —namely, 
propagated?  by  conduction  and  by  radiation.  Heat  is  "propa- 
gated by  conduction  when  it  passes  from  one  substance  to  another 
in  contact  with  it.  Heat  is  propagated  by  radiation  when  it 
passes  through  the  air,  or  any  other  elastic  fluid.  (See  par.  1469.) 
720.  Different  bodies  conduct  heat  with  differ- 
What  are  the  ent  degrees  of  facility.  The  metals  are  the  best 
orsofheat*  conductors;  and  with  regard  to  their  conducting 
power,  stand  in  the  following  order,  namely:  Gold, 
platinum,  silver,  copper,  iron,  zinc,  tin,  lead. 

721.  Any  liquid,  therefore,  may  he  more  readily  heated  in  a 
•diver  vessel  than  in  any  other  of  the  same  thickness,  except  one  oJ 
gold,  or  of  platinum,  on  account  of  its  great  conducting  power. 

Why  are  the        722.  Metals,    on    account    of  their    conducting 
handles  of  tea  power,  cannot  be  handled  when  raised  to  a  tempe- 

made°'ofwoodi rature  above  12°  degrees  of  Fahrenheit.  For  this 
reason,  the  handles  of  metal  tea-pots  and  coffee- 
pots are  commonly  made  of  wood ;  since,  if  they  were  made  of 
metal,  they  would  become  too  hot  to  be  grasped  by  the  hand, 
soon  after  the  vessel  is  filled  with  heated  fluid. 

723.  Wood   conducts   heat  very   imperfectly.     For   this  reason, 
wooden  spoons  and  forks  are  preferred  for  ice.     Indeed,  so  imper- 
fect a  conductor  of  heat  is  wood,  that  a  stick  of  wood  may  be  grasped 
by  the  hand  while  one  end  of  the  stick  is  a  burning  coal.     But  an 
iron  bar,  being  a  good  conductor  of  heat,  cannot  be  handled  near 
the  heated  end. 

724.  Animal  and  vegetable  substances,  of  a  loose  texture,  such 
as  fur,  wool,  cotton,  &c.,  conduct  heat  very  imperfectly  ,  hence  their 
efficacv  in   preserving   the  warmth  of  the    body.     Water   becomes 
scalding  hot  at  150  degrees ;   but  air,  heated  far  beyond  the  tempe- 
rature of  boiling  water,  may  be  applied  to  the  skin  without  much 
pain.     Sir  Joseph  Banks,  with  several  other  gentlemen,  remained 
some  time  in  a  room  when  the  heat  was  52 D  above  the  boiling 
point ;  but  though  they  could  bear  the  contact  of  the  heated  air 
they  could  not  touch  any  metallic  substance,  as  their  watch-chains 


PYKOJNOMIOS.  1'Ji 

money,  <fcc.  Eggs,  placed  on  a  tin  frame,  were  roasted  hard  in 
twenty  minutes  ;  and  a  beef-steak  was  overdone  in  thirty-three 
minutes. 

725.  Chantrey,  the  celebrated  sculptor,  had  an  oven  which  ne 
used  for  drying  his  plaster  cuts  and  moulds.     The  thermometer  gen- 
arally  stood  at  300  degrees  in  it,  yet  the  workmen  entered,  and 
remained  in  it  some  minutes  without  difficulty  ;  but  a  gentleman 
rmce  entering  it  with  a  pair  of  silver-mounted  spectacles  on,  had  his 
face  burnt  where  the  metal  came  in  contact  with  the  skin. 

726.  The  air,  being  a  bad  conductor,  never  radiates  heat,  nor  is 
it  ever  made  hot  by  the  direct  rays  of  the  sun      The  air  which  comes 
in  contact  with  the  surface  of  the  earth  ascends,  and  warms  the  air 
through  which  it  passes  in  its  ascent.     Other  air,  heated  in  the 
same  way,  also  ascends,  carrying  heat,  and  this  process  is  repeated 
till  all  the  air  is  made  hot. 

727.  In  like  manner,  in  cold  weather,  the  air  resting  on  the  earth 
ts  made  cold  by  contact.     This  cold  air  makes  the  air  above  it  cold, 
and  cold  currents  (or  wind)  agitate  the  mass  together  till  a  uniform 
temperature  is  produced. 

728.  Heat  is  reflected  by  bright  substances,  and 
How  is  heat       .  ,        n       n  .11    t 

reflected?         *ne  angie  of  reflection  will   be  equal  to  th*  anglf 

of  incidence. 

729.  Advantage  has  been  taken  of  this  property  of  heat  in  the 
construction  of  a  simple   apparatus   for  baking.     It-is  a  bright  tin 
case,  having  a  cover  inclined  towards  the  fire  in  such  a  manner  as 
to  reflect  the  heat  downwards.     In  this  manner  use  is  made  botli  ot 
the  direct  heat  of  the  fire,  and  the  reflected  heat,  which  would  other- 
wise pass  into  the  room.     The  whole  apparatus,  thus  connected  with 
the  culinary  department,  is  called,  in  New  England,  "  The  Connect- 
icut baker." 

730.  This   power  of  reflecting   heat,  possessed  by   bright   sub- 
stances, is  the  reason  why  andirons  and  other  articles  that  are  kept 
bright,  although   standing   very  near  the  fire,  u^ver  become  hot; 
while  other  darker  substances,  further  from  the  fire,  become  hot. 
But,  if  they  are  not  bright,  heat  will  penetrate  tLein. 

731.  The  reflecting  power  of  bright  and  light  colovod  substances 
accounts  also  for  the  superior  coolness  of  whi^e  and  light-colored 
fabrics  for  clothing. 

Whti  are  dark  ^^'  Black  an(^  dark-coloied  surfaces  absorb 
garments  heat.  This  is  the  reason  why  black  and  dark- 

warmer  than  colore(}  fabrics  are  warmer  when  made  into  ear- 
light  ones  ?  . 

ments  than  those  ol  light  color. 

733.  Snow  or  ice  will  melt  under  a  piece  of  black  cloth,  while 
it  would  remain  perfectly  solid  under  a  white  one.  The  farmers  in 
some  of  the  mountainous  parts  of  Europe  are  accustomed  to  spread 


192  NATURAL    PHILOSOPHY. 

black  earth,  or  soot,  over   the  snow,  in  the  spring,  to  hasten  its 
welting,  and  enable  them  to  commence  ploughing. 

What  effect  has  heat     734    rrhe  density  of  all  substances  is  aug- 

upon  the  density  of  ,   .,.     .    . 

tubstances?  mented  by  cold,  and  diminished  by  neat. 

There  is  a  remarkable  exception  to  this  remark,  and  that  is  in  tho 
Cd,<?e  of  water  ;  which  instead  of  contracting,  expands  at  the  freez- 
ing point,  or  when  it  fc  frozen.  This  is  the  reason  why  pitchers, 
ihd  other  vessels,  containing  water  and  other  similar  fluids,  are  so 
often  broken  when  the  liquid  freezes  in  them.  For  the  same  reason, 
ice  floats  instead  of  sinking  in  water  ;  for,  as  its  density  is  dimin- 
ished, its  specific  gravity  is  consequently  diminished.  Were  it  not  for 
this  remarkable  property  of  water,  large  ponds  and  lakes,  expose' 
to  intense  cold,  would  become  solid  masses  of  ice  ;  for.  if  the  ice, 
when  formed  on  the  surface,* were  more  dense  (that  is,  more  heavj) 
than  the  water  below,  it  would  sink  to  the  bottom,  and  the  water 
above,  freezing  in  its  turn,  would  also  sink,  until  the  whole  body 
of  the  water  would  be  frozen.  The  consequence  would  be  the  total 
destruction  of  all  creatures  in  the  water.  But  the  specific  gravity 
of  ice  causes  it  to  continue  on  the  surface,  protecting  the  water 
below  from  congelation. 

735.  Cold  is  merely  the  absence  of  heat ;  or  rather. 
What  is 
co/rf?       more   properly  speaking,  inferior  degrees  of  heat  are 

termed  cold.     (See  par.  1463.) 

736.  The  effect  of  heat  and  cold,  in  the  expansion  and  contrac 
tion  of  glass,  is  an  object  of  common  observation  ;  for  it  is  this 
expansion  and  contraction  which  cause  so  many  accidents  with  glasb 
articles.     Thus,  when  hot  water  is  suddenly  poured  into  a  cold  glass 
of  any  form,  the  glass,  if  it  have  any  thickness,  will  crack  ;  ano 
on  the  contrary,  if  cold  water  be  poured  into  a  heated  glass  vessel 
the  same  effect  will  be  produced.     The  reason  of  which  is  this  ; 
Heat  makes  its  way  but  slowly  through  glass ;  the  inner  surface, 
therefore,  when  the  hot  water  is  poured  into  it,  becomes  heated, 
and,  of  course,  distended  before  the  outer  surface,  and  the  irregular 
expansion  causes  the  vessel  to  break.     There  is  less  danger  of  frao- 
ture,  therefore,  when  the  gjass  is  thin,  because  the  heat  readily  pen- 
etrates it,  and  there  is  no  irregular  expansion. 

737.  The  glass  chimneys,  used  for  oil  and  gas  burners,  are  often 
broken  by  being  suddenly  placed,  when  cold,  over  a  hot  flame.    The 
danger  of  fracture  may  be  prevented   (it  is  said)  by  making  a  mi- 
nute notch  on  the  bottom  of  the  tube  with  a  diamond.     This  precau- 
tion has  been  used  in  an  establishment  where  six  lamps  were  lighted 
ttvery  day,  and  not  a  single  glass  has  been  broken  in  nine  years 

What  bodies  retain      738.  Different  bodies  require  different  quan- 
heat  the  longest  ?      tides  of  heat  to  raise  them  to  the  same  tern- 


PYRONOMICS  193 

perature  ;  and  those  which  are  heated  with  most  difficulty  retain 
their  heat  the  longest. 

Thus,  oil  becomes  heated  more  speedily  than  water,  and  it 
likewise  cools  more  quickly.  (See  par.  1471.) 

739.  The  most  obvious  and  direct  effect  of  heat  on  a  bodj 
is  to  increase  its  extension  in  all  directions. 

740.  Coopers,  wheelwrights  and  other  artificers,  avail  themselves 
of  this  property  in  fixing  iron  hoops  on  casks,  and  the  tires  or  irons 
on  wheels.     The  hoop  or   tire,  having  been  heated,  expands,  and, 
being  adapted  in  that  state  to  the  cask  or  the  wheel,  as  the  metal 
contracts  in  cooling  it  clasps  the  parts  very  firmly  together. 

741.  From  what  lias  been  stated  above,  it  will  be  seen  that  an 
allowance  should  be   made  for  the  alteration   of  the  dimensions  in 
metallic  beams  or  supporters,  caused  by  thedilatation  and  contraction 
effected  by  the  weather.     In  the  iron  arches  of  Southwark  Bridge, 
over  the  Thames,  the  variation  of  the  temperature  of  the  air  causes 
a  difference  of  height,  at  different  times,  amounting  to  nearly  an  inch. 

A  happy  application  of  the  expansive  power  of  heat  to  the  mechanic 
arts  was  made  some  years  ago,  at  Paris.  The  weight  of  the  roof  of  a 
building,  in  the  Conservatory  of  Arts  and  Trades,  had  pressed  out- 
wards the  side  walls  of  the  structure,  and  endangered  its  security 
The  following  method  was  adopted  to  restore  the  perpendicular 
direction  of  the  structure.  Several  apertures  were  made  in  the 
walls,  opposite  to  each  other,  through  which  iron  bars  were  intro- 
duced, which,  stretching  across  the  building,  extended  beyond  the 
outside  of  the  walls.  These  bars  terminated  in  screws,  at  each  end, 
to  which  large  broad  nuts  were  attached.  Each  alternate  bar  was 
then  heated  by  means  of  powerful  lamps,  and  their  lengths  being  thus 
increased,  the  nuts  on  the  outside  of  the  building  were  screwed  up 
close  to  it,  and  the  bars  were  suffered  to  cool.  The  powerful  con- 
traction of  the  bars  drew  the  walls  of  the  building  closer  together 
and  the  same  process  being  repeated  on  all  the  bars,  the  walls  were 
gradually  and  steadily  restored  to  their  upright  position. 

742.  The   Pyrometer  is  an   instrument  to 
What    is    the      ,          ^  .          „  ,     ,.        ,        ,  .. 

Pyrometer]       show  the  expansion  oi   bodies   by  the  applica- 
tion of  heat. 

It  consists  of  a  metallic  bar  or  wire,  with  an  index  connected 
with  one  extremity.  On  the  application  of  heat,  the  bar  expands, 
and  turns  the  index  to  show  the  degree  of  expansion. 

743.  Wedgewood's  pyrometer,  the  instrument  commonly  used 
for  high  temperatures,  measures  heat  by  the  contraction  of 
clay. 


194  NATURAL    PHILOSOPHY. 


What  effect  ^^  ^e  exPansi°n  caused  by  heat  i.i 
has  heat  on  solid  and  liquid  bodies  differs  in  different  sub- 
^  ITthe  stances  '  bu*  agriform  fluids  all  expand  alike, 
w/idf,  liquid  and  and  undergo  uniform  degrees  of  expansion  at 
triform  state?  ^.^  temperatujm 

745.  The  expansion  of  solid  bodies  depends,  in  some  degree,  on 
the  cohesion  of  their  particles  ;  but,  as  gases  and  vapors  are  desti- 
tute of  cohesion,  heat  operates  on  them  without  any  opposing  power. 

746.  When  heat  IP.  applied  to  water  or  other 
What  effect  liquids,  it  converts  them  into  steam  or  vapor.  The 
the  f  f  deprivation  of  heat  reconverts  them  into  the  liquid 

liquid  bodies  ?  form.     It  is  on  this  principle  that  distillation  takes 

place. 

What  is  a  747.  The  vessel  employed  for  distillation  is  called 
***?'  a  Still.* 

Fig.  107. 


Explain  748.  Fig.  107  represents  a  Still.  A  liquid  being 
Fig.  107.  poured  into  the  large  vessel  a,  heat  is  applied  below, 
which  converts  the  liquid  gradually  into  steam  or  vapor,  which, 
having  no  other  outlet,  passes  through  the  spiral  tube,  called  the 
worm,  in  vessel  &,  and  from  5  through  another  worm,  in  c.  The 
worm,  being  surrounded  with  cold  water,  condenses  the  vapor  in 
the  tube  or  worm,  and  reconverts  it  to  a  fluid  state,  and  it  flows 

*  The  subject  of  distillation  properly  belongs  to  the  science  of  Chemistry, 
but  it  is  here  introduced  for  the  benefit  of  those  who  cannot  readily  refer 
to  a  treatise  on  that  subject. 


PYKOKOMICS.  195 

out  at  e  in  a  tepid  stream.  The  worm  is  of  different  lengths,  and 
its  only  use  is  to  present  a  large  extent  of  surface  to  the  cold 
water,  so  that  the  vapor  may  readily  be  condensed. 

749.  The  process  of  distillation  is  sometimes  used  to  purify  a 
liquid,  as  the  vapors  which  rise  are  unmixed  with  the  impurities  of 
the  fluid.  Important  changes  are  thus  made,  and  the  still  becomes 
highly  useful  in  the  arts. 

At  what  tem-        750.  When  water  is  raised  to  the  tempera- 

w^Tonvert-  ture  of  212°  of  Fahrenheit's  thermometer,  it 
ed  into  steam  ?  is  converted  into  steam.  It  is  then  highly 
elastic  and  compressible. 

What  effect  ^^'  ^e  elastic  force  of  steam  is  increased 
has  heat  upon  by  heat ;  and  decrease  of  heat  diminishes  it. 
The  amount  of  pressure  which  steam  will  exert 
depends,  therefore,  on  its  temperature. 

^^  .  752.  The  temperature  of  steam  is  always  the 

temperature  same  with  that  ..  of  the  liquid  from  which  it  is 
of  confined  formed,  while  it  remains  in  contact  with  that  liquid , 
and  when  heated  to  a  great  degree,  its  elastic  force 
will  cause  the  vessel  in  which  it  is  contained  to  burst,  unless  Jt 
is  made  sufficiently  strong  to  resist  a  prodigious  pressure. 

753.  It  has  already  been  stated  that  water  is  converted  into 
steam  at  the  temperature  of  212°.  When  closely  confined  it  may  be 
raised  to  a  higher  temperature,  and  it  will  then  emit  steam  ol 
greatly  increased  elastic  force. 

How  is  steam  754:.  When  any  portion  of  steam  comes  in 
condensed?  contact  with  water,  it  instantly  parts  with  its 
heat  to  the  water,  and  becomes  condensed  into  water.  The 
whole  mass  then  becomes  water,  increased  in  temperature 
by  the  amount  of  heat  which  the  steam  has  lost. 
On  what  property  755.  This  is  the  great  and  peculiar 
tSW^  P«Perty  of  steam,  on  which  its  me- 
depend?  chanical  agencies  depend  —  namely,  its 

power  of  exerting  a  high  degree  of  elastic  force,  and  losing 
it  instantaneously. 


196  NATURAL    PHILOSOPHY 


How  may  the  ^^'  There  are  two  ways  in  which  steam 
mechanical  is  made  instantly  to  lose  its  mechanical  force  ; 
force  of  steam  ,  *  '-  f  n  i 

le  instantly     namely,  first,  by  suddenly  opening  a   passage 

destroyed?  for  j^s  escape  into  the  open  air,  where  it  iinnift- 
iiately  becomes  visible,*  by  a  sudden  loss  of  part  of  its  heat, 
tfhich  it  gives  to  the  air  ;  and  secondly,  by  conveying  it  to 
i  vessel  called  a  condenser,  where  it  comes  directly  into 
s.tmtact  with  a  stream  of  water,  to  which  it  instantly  gives 
up  its  heat  and  is  condensed  into  water. 

757.   Steam  occupies  a  space  about  seven- 
What  space 
does  steam       teen  hundred  times  larger  than  when  it  is  con- 

occupy  f  verted  into  water.  But  the  space  th&t  a  given 
quantity  of  water  converted  into  steam  will  occupy  depends 
upon  the  temperature  of  the  steam.  The  more  it  is  heated 
the  greater  space  it  will  fill,  and  the  greater  will  be  its 
expaj»sive  force. 

WJiaf  is  the  758.    THE     STEAM-ENGINE.  —  The     Steam- 

Steam-engme  ?  engjne  js  a  machine  moved  by  the  expansive 
force  of  steam. 

In  what  man-  759.  The  mode  in  which  steam  is  made  to  act 
ner  is  steam  is  by  causing  its  expansive  force  to  raise  a  solid 
made  to  act  ?  piston  accurately  fitted  to  the  bore  Of  a  cylinder, 

like  that  in  the  forcing-pump.  The  piston  rises  by  the  impulse 
of  expanding  steam,  admitted  into  the  cylinder  below.  When 
the  piston  is  thus  raised,  if  the  steam  below  it  be  suddenly  con- 
densed by  the  admission  of  cold  water,  or  withdrawn  from  under 

*  Steam  in  a  highly  elastic  state  —  that  is,  when  at  a  high  temperature  — 
Is  perfectly  dry  and  invisible.  The  reason  that  we  are  able  to  see  it  after  it 
has  performed  its  work  and  issues  from  the  steam-engine  is,  that  as  soon  as  it 
eoines  in  contact  with  the  air  it  immediately  parts  with  a  portion  of  its 
heat  (and,  because  air  is  not  a  good  conductor,  only  a  portion),  and  is  con 
densed  into  small  vesicles,  which  present  a  visible  form,  resembling  smoke 
Its  expansive  force,  however,  is  not  wholly  destroyed;  for  the  vesicles  them 
selves  expand  as  they  rise,  and  soon  become  invisible,  mingling  with  other 
vapors  in  the  air.  Could  we  look  into  the  cylinder,  filled  with  highly  elastic 
steam,  we  should  be  able  to  see  nothing.  But,  that  the  steam  is  thcve,  and 
ir.  its  invisible  form  exerting  a  prodigious  force,  we  know  by  the  movement? 
of  the  piston 


STEAM-ENGINE.  197 

it,  a  vacuum  will  be  formed,  and  the  pressure  of  the  atmosphere 
on  the  piston  above  will  drive  it  down.  The  admission  of  more 
steam  below  will  raise  it  again,  and  thus  a  continued  motion  ot 
the  piston,  up  and  down,  will  be  produced.  This  motion  of  the 
piston  is  communicated  to  wheels,  levers,  and  other  machinery, 
in  such  a  manner  as  to  produce  the  effect  intended. 

760.  This  is  the  mode  in  which  the  engine  of 
How  was  the 

steam-engine       Thomas  Newcomen,  commonly  called  the  atmos- 

of  Neiccomen  pheric  engine,  was  constructed.  It  was  called 
constructed?  .  . 

the  atmospheric  engine  because  all  of  the  work 

was  done  by  the  pressure  of  the  atmosphere — namely,  in  the 
downward  motion  of  the  piston. 

761.  The   celebrated  James  Watt  introduced 
What  improve- 
ments did  Watt two  important    improvements    into  the    steam- 

make  in  the  engine.  Observing  that  the  cooling  of  the  cylin- 
steam-cngine  ?  ,  .  ,_  .  .  ..  .,_ 

der  by  the  water  thrown  into  it  to  condense  the 

steam  lessened  the  expansibility  of  the  steam,  he  contrived  a 
method  to  withdraw  the  steam  from  the  principal  cylinder,  after 
it  had  performed  its  office,  into  a  conden sing-chamber,  where  it 
is  reconverted  into  water,  and  conveyed  back  to  the  boiler.  The 
other  improvement,  called  the  double  action,  consists  in  substitut- 
ing the  expansive  power  of  steam  for  the  atmospheric  pressure. 
This  was  performed  by  admitting  the  steam  into  the  cylinder 
above  the  raised  piston  at  the  same  moment  that  it  is  removed 
from  Mow  it ;  and  thus  the  power  of  steam  is  exerted  in  the  de- 
scending as  well  as  in  the  ascending  stroke  of  the  piston  ;  and  a 
much  greater  impetus  is  given  to  the  machinery  than  by  the 
former  method.  From  the  doulle  action  of  the  steam  above  as 
well  as  Mow  the  piston,  and  from  the  condensation  of  the  steam 
after  it  has  performed  its  office,  this  engine  is  called  Watt's 
double-acting  condensing  steam-engine.  [See  also  No.  766.] 

Explain  762.  Fig.  108  represents  that  portion  of  the  steam- 
Fig.  108.  engine  in  which  steam  is  made  to  act,  and  propel  such 
machinery  as  may  be  connected  with  it.  It  also  exhibits  two 


NATURAL    PHILOSOPHY. 


improvements  of  Mr.  Watt. 
The  principal  Darts  are  the 
noiler,  the  cylinder  and  its 
piston,  the  condenser,  the 
air-pump,  the  steam-pipe, 
the  eductiou-pipe,  and  the 
cistern.  In  this  figure,  A 
represents  the  boiler,  G 
the  cylinder,  with  H  the 

piston,  B  the  steam-pipe,  with  two  branches*  communicating 
with  the  cylinder,  the  one  above  and  the  other  below  the  piston. 
This  pipe  has  two  valves,  F  and  G,  which  are  opened  and  -closed 
alternately  by  machinery  connected  with  the  piston.  The  steam 
is  carried  through  this  pipe  by  the  valves,  when  open,  to  the 
cylinder,  both  above  and  below  the  piston.  K  is  the  eduction- 
pipe,  having  two  branches,  like  the  steam-pipe,  furnished  with 
valves,  &c.,  which  are  opened  and  shut  by  the  same  machinery. 
By  the  eduction-pipe  the  steam  is  led  off  from  the  cylinder,  as 
1-he  piston  ascends  and  descends. 

L  is  the  condenser,  and  0  a  stop-cock  for  the  admission  of  cold 
water.  M  is  the  pump.  N  is  the  cistern  of  cold  water  in  which 
the  condenser  is  immersed.  R  is  the  safety-valve.  When  the 
valves  are  all  open,  the  steam  issues  freely  from  the  boiler,  and 
circulates  through  ail  the  parts  of  the  machine,  expelling  the 
air.  This  process  is  called  blowing  out,  and  is  heard  when  a 
steamboat  is  about  starting. 

Now,  the  valves  F  and  Q  being  closed,  and  G  and  P  remain- 
ing open,  the  steam  presses  upon  the  piston  and  forces  it  down 
As  it  descends,  it  draws  with  it  the  end  of  the  working-beam, 
which  is  attached  to  the  piston-rod  J  (but  which  is  not  repre 
sented  in  the  figure).  To  this  working-beam  (which  is  a  lever 
of  the  first  kind)  bars  or  rods  are  attached,  which,  rising  and 
falling  with  the  beam  and  the  piston,  open  the  stop-cock  0,  ad- 

*  The  steam  and  the  eduction  pipes  are  sometimes  made  in  form?  differing 
from  those  in  the  figure,  and  they  differ  much  in  different  tmgines. 


STEAM-ENGINE.  199 

initting  a  stream  of  cold  water,  which  meets  the  steam  from  the 
cylinder  and  condenses  it,  leaving  no  force  below  the  piston  to 
oppose  its  descent.  At  this  moment  the  rods  attached  to  the 
working-beam  close  the  stop-cocks  Gr  and  P,  and  open.  F  and  Q. 
The  steam  then  flows  in  below  the  piston,  and  rushes  from  above 
it  into  the  condenser,  by  which  means  the  piston  is  forced  up 
again  with  the  same  power  as  that  with  which  it  descended. 
Thus  the  steam-cocks  Gr  and  P  and  F  and  Q  are  alternately 
opened  and  closed  ;  the  steam  passing  from  the  boiler  drives  the 
piston  alternately  upwards  and  downwards,  and  thus  produces 
a  regular  and  continued  motion.  This  motion  of  the  piston, 
being  communicated  to  the  working-beam,  is  extended  to  other 
machinery,  and  thus  an  engine  of  great  power  is  obtained. 

The  pump  M,  the  rod  of  which  is  connected  with  the  working- 
beam,  carries  the  water  from  the  condenser  back  into  the  boiler, 
by  a  communication  represented  in  Fig.  109. 

The  safety-valve  R,  connected  with  a  levej*  of  the  second 
kind,  is  made  to  open  when  the  pressure  of  the  steam  within  the 
boiler  is  too  great.  The  steam  then  rushing  through  the  aperture 
under  the  valve,  removes  the  danger  of  the  bursting  of  the  boiler. 

How  is  the  7^3.  The  power  of  a  steam-engine  is  gen- 

power  of  a  ,,  111 

steam-engine     erally   expressed   by  the  power  ot   a   horse, 

estimated?  which  can  raise  33,000  Ibs.  to  the  height  of 
one  foot  in  a  minute.  An  engine  of  100  horse  power  is 
one  that  will  raise  3,300,000  Ibs.  to  the  height  of  one  foot 
in  one  minute. 

What  are  the         764.  The  steam-engine  is    constructed  in    va- 
r^ous    forms,  and  no  two  manufacturers   fol  low- 


one?  how  do  they  ing  exactly  the  same  pattern  ;  but  the  two  priri- 
dijferi  c-paj  kjnc[s  are  the  high  and  the  low  pressure 

engines,  or,  as  they  are  sometimes  called,  the  non-condensing  and 
the  condensing  engines.  The  non-condensing  or  high-pressure, 
engines  differ  from  the  low-pressure  or  condensing  engines  in 
having  no  condenser.  The  steam,  after  having  moved  the  piston, 


200  NATURAL    PHILOSOPHY. 


is  let  off  into  the  open  air.  \s  this  ki-nd  of  engine  occupies  Jese 
space,  and  is  much  less  complicated,  it  is  generally  used  on  rail- 
roads. In  the  tow-pressure  or  condensing  engines,  the  steam, 
after  having  moved  the  piston,  is  condensed,  or  converted  into 
water,  and  then  conducted  back  into  the  boiler. 

765.  The  steam-engine,  as   it   is   constructed 
Who  were  the  . 

principal   im-    at  tae  present  day,  is  the  result  of  the  inventions 

provers  of  the  and  discoveries  of  a  number  of  distinguished  indi- 
iteam-mgine?  vidualgj  at  different  periods.  Among  thosa  who 
have  contributed  to  its  present  state  of  perfection,  and  its  ap- 
plication to  practical  purposes,  may  be  mentioned  the  names  o^ 
Somerset,  the  Marquis  of  Worcester,  Savery,  Newcomeu,  Fulton, 
and  especially  Mr.  James  Watt. 

766.  To  the  inventive  genius  of  Watt  tne  engine  is  indebted  foi 
the  condenser,  the  appendages  for  parallel  notion-*  the  application  <»f 
the  governor,  and  for  the  double  action.  In  the  words  of  Mr.  Jeffrey,, 
it  may  be  added,  that,  "  by  his  admirable  contrivances,  and  those  of 
Mr.  Fulton,  it  has  become  a  thing  alike  stupendous  for  its  force  and 
its  flexibility ;  for  the  prodigious  power  it  can  exert,  and  the  ease 
and  precision  and  ductility  with  which  it  can  be  varied,  distributed, 
and  applied.  The  trunk  of  an  elephant,  that  can  pick  up  a  pin,  or 
rend  an  oak,  is  as  nothing  to  it.  It  can  engrave  a  seal,  and  crush 
masses  of  obdurate  metal  before  it;  draw  out,  without  breaking,  a 
thread  as  fine  as  gossamer,  and  lift  up  a  ship  of  war  like  a  baublo 
in  the  air.  It  can  embroider  muslin,  and  forge  anchors  ;  cut  steel 
into  ribands,  and  impel  loaded  vessels  against  the  fury  of  the  winds 
and  waves." 

Explain  767.  Fig.  109  represents  Watt's  double-acting  condens- 
Fig.  109.  jng  steam_engine,  in  which  A  represents  the  boiler,  con- 
taining a  large  quantity  of  water,  which  is  constantly  replaced  as 
fast  as  portions  are  converted  into  steam.  B  is  the  steam-pipe, 
conveying  the  steam  to  the  cylinder,  having  a  steam-cock  b  to 
admit  or  exclude  the  steam  at  pleasure. 

C  is  the  cylinder,  surrounded  by  the  jacket  c  c,  a  space  kept 
constantly  supplied  with  hot  steam,  in  order  to  keep  the  cylinder 
from  being  cooled  by  the  external  air.  D  is  the  eduction-pipe, 
communicating  between  the  cylinder  and  the  condenser.  E  is 
the  condenser,  with  a  valve  e  called  the  injection-cock,  admitting 


STEAM-ENGINE. 

a  jet  of  cold  water,  which  meets  the  steam  the  instant  that  tho 
steam  enters  the  condenser.  F  is  the  air-pump,  which  is  a  com- 
mon suction-pump,  but  is  here  called  the  air-pump  because  it 
removes  from  the  condenser  not  only  the  water,  but  also  the  air, 
and  the  steam  that  escapes  condensation.  G  G  is  a  cold-water 
cistern,  which  surrounds  the  condenser,  and  supplies  it  with  cold 
water,  being  filled  by  the  cold-water  pump,  which  is  represented 

Fig.  109. 


by  H.  I  is  the  hot  well,  containing  water  from  the  condenser ; 
K  is  the  hot-water  pump,  which  conveys  back  the  water  of  con- 
densation from  the  hot  well  to  the  boiler. 

L  L  are  levers,  which  open  and  shut  the  valves  in  the  chan- 
nel between  the  steam-pipe,  cylinder,  eduction-pipe,  and  con- 
denser ;  which  levers  are  raised  or  depressed  by  projections 
attached  to  the  piston-rod  of  the  pump.  M  M  is  an  apparatus 
for  changing  the  circular  motion  of  the  w'orking-beani  into  par- 


202 


NATURAL    PHILOSOPHY. 


STEAM-ENGINE.  tiOH 

aliel  motion,  so  that  the  piston-rods  are  made  to  move  in  a  straight 
line  N  N  is  the  working-beam,  which,  being  moved  by  the 
rising  and  falling  of  the  piston  attached  to  one  end,  coinmuoi- 
cates  motion  to  the  fly-wheel  by  means  of  the  crank  P,  and  from 
the  fly-wheel  the  motion  is  communicated  by  bands,  wheels  or 
levers,  to  the  other  parts  of  the  machinery.  0  0  is  the  governor. 

The  governor,  being  connected  with  the  fly-wheel,  is  made  to 
partake  of  the  common  motion  of  the  engine,  and  the  balls  will 
remain  at  a  constant  distance  from  the  perpendicular  shaft  so 
long  as  the  motion  of  the  engine  is  uniform  ;  but,  whenever  the 
engine  moves  faster  than  usual,  the  balls  will  recede  further  froni 
the  shaft,  and  by  partly  closing  a  valve  connected  with  the 
toiler,  will  diminish  the  supply  of  steam  to  the  cylinder,  and 
thus  reduce  the  speed  to  the  rate  required. 

The  ^team-engine  thus  constructed  is  applied  to  boats  to, turn 
wheels  having  paddles  attached  to  their  circumference,  which 
answer  the  purpose  of  oars.  [See  Fig.  110.]  It  is  used  also 
in  work-shops,  factories,  &c. ;  and  different  directions  and  veloc- 
ities may  be  given  to  the  motion  produced  by  the  action  of  the 
steam  on  the  piston,  by  connecting  the  piston  to  the  beam  with 
wheels,  axles  and  levers,  according  to  the  principles  stated 
under  the  head  of  Mechanics. 

Steamboats  are  used  principally  on  rivers,  in  harbors,  bays,  and  on 
the  coast.  They  are  made  of  all  sizes,  and  carry  engines  of  different 
power,  proportioned  to  the  size  of  the  boat. 

The  steamship  [See  Fig.  Ill],  in  addition  to  its  steam -engires 
Fig.  111. 


^04  NATUBAL   PHILOSOPHY. 

•nd  paddles,  is  rigged  with  masts  and  sails  to  increase  the  speed.  01 
to  make  progress  if  the  engines  get  out  of  order. 

The  Propeller  differs  from  a  steam-boat  or  steam-ship,  by  having 
an  immense  screw  projecting  from  under  the  stern  of  the  ship,  instead 
of  paddle-wheels.  The  screw  is  caused  to  revolve  by  means  of  steam- 
engines,  and  forces  the  vessel  forward  by  its  action  on  the  water. 


What  is  the  ^^*  ^e  locomotive  engine  is  a  high- 
locomotive  pressure  steam-engine,  mounted  on  wheels, 
steam-engine?  ^  uged  t()  draw  1()adg  Qn  ft  rajiroa(J)  CI  othel 

level  road.  It  is  usually  accompanied  by  a  large  wagon. 
called  a  tender,  in  which  the  wood  and  water  used  by  the 
engine  are  carried. 

Explain  769.  Fig.  112  represents  a  side  view  of  the  internal 
Fig.  112.  construction  of  a  locomotive  steam-engine  ;  in  which 
F  represents  the  fire-box,  or  place  where  the  fire  is  kept;  D 
the  door  through  which  the  fuel  is  introduced.  The  spaces 
marked  B  are  the  interior  of  the  boiler,  in  which  the  water 
stands  at  the  height  indicated  by  the  dotted  line.  The  boiler  is 
closed  on  all  sides,  all  its  openings  being  guarded  by  valves. 
The  tubes  marked  p  p  conduct  the  smoke  and  flame  of  the  fuel 
through  the  boiler  to  the  chimney  C  C,  serving,  at  the  same 
time,  to  communicate  the  heat  to  the  remotest  part  of  the  boiler. 
By  this  arrangement,  none  of  the  heat  is  lost,  as  these  tubes  are 
all  surrounded  by  the  water.  S  S  S  is  the  steam  -pipe,  open  at 
the  top  V  S,  having  a  steam-tight  cock,  or  regulator,  V,  which 
is  opened  and  shut  by  the  lever  L,  extending  outside  of  tbe 
boiler,  and  managed  by  the  engineer. 

The  operation  of  the  machine  is  as  follows:  The  steam  being 
generated  in  great  abundance  in  the  boiler,  and  being  unable  to 
escape  out  of  it,  acquires  a  considerable  degree  of  elastic  force. 
If  at  that  moment  the  valve  V  be  opened,  by  the  handle  L,  the 
steam,  entering  the  pipe  S,  passes  in  the  direction  of  the  arrow, 
through  the  tube,  and  enters  the  valve-box  at  X.  There  a 
tsliding-valve,  which  moves  at  the  same  time  with  the  machine, 
opens  for  the  steam  a  communication  successively  with  each  end 
of  the  cylinder  below.  Thus,  in  the  figure,  the  entrance  on  the 
right  hand  of  the  sliding-valve  is  represented  as  being  open,  and 


STEAM-ENGINE. 


205 


206 


NATUEAL   PHILOSOl'HX 


STEAM  KNGINE. 


207 


NATUKAL    PHILOSOPHY. 


STEAM-ENGINE.  fr* 

the  steam  follows  in  the  direction  of  the  arrows  into  thecyliiuw. 
where  its  expansive  force  will  move  the  piston  P  in  the  direc- 
tion of  the  arrow.     The  steam  or  air  on  the  other  side  of  the 
piston  passes  out  in  the  opposite  direction,  and  is  conveyed  by  a 
tube  passing  through  C  C  into  the  open  air. 

The  motion  of  the  piston  in  the  direction  of  the  arrow  causes 
the  lever  N  to  close  the  sliding-vah  e  on  the  right,  and  open  a 
communication  for  the  steam  on  the  opposite  side  of  the  piston 
P,  where  it  drives  the  piston  back  towards  the  arrow,  at  the 
same  time  affording  a  passage  for  the  steam  on  the  right  of  the 
piston  to  pass  into  the  open  air. 

Motion  being  thus  given  to  the  piston,  it  is  communicated,  by 
means  of  the  rod  R  and  the  beam  G,  to  the  cranks  K  K,  which, 
being  connected  with  the  axle  of  the  wheel,  causes  it  to  turu, 
and  thus  move  the  machine. 

Thus  constructed,  and  placed  on  a  railroad,  the  locomotive 
Bteam-engine  is  advantageously  used  as  a  substitute  for  horse 
power,  for  drawing  heavy  loads. 

The  apparatus  of  safety-valves,  and  other  appliances  for  the 
management  of  the  power  produced  by  the  machine,  are  the 
same  in  principle,  though  differing  in  form,  with  those  used  in 
other  steam-engines;  for  a  particular  description  of  which,  the 
student  is  referred  to  practical  treatises  upon  the  subject. 

770.  THE  STATIONARY  STEAM-ENGINE.- 


-    *j, 

ts  tne 
best  form  of    This  engine  is  generally  a  high-pressure  or 

thema*Jteam~€n~  non-condensing  engine,  used  to  propel  ma- 
chinery in  work-shops  and  factories.  As  it  is 
designed  for  a  labor-saving  machine,  it  is  desirable  to  com- 
bine simplicity  arid  economy  with  safety  and  durability  in 
its  construction  ;  and  that  form  of  this  engine  is  to  be  pre- 
ferred which  in  the  greatest  degree  unites  these  qualities. 

Describe  the  Sf  a-        771.    The   figure   on   page    207    represents 

tionary    Steam-    Tufts'  stationary  steam-engine,    with  sections  ol 

the  interior.     Like  the  double-acting  condens- 


NATURAL   PHILOSOPHY. 

ing  engine  of  Mr.  Watt,  desci  ibed  in  Fig.  109,  it  is  furnished 
with  a  governor,  by  which  the  supply  of  steam  is  regulated  • 
and,  like  the  locomotive,  Fig.  112,  the  cylinder,  with  its  piston, 
has  a  horizontal  position.  The  steam  is  admitted  into  the  valve- 
box  through  an  aperture  at  E,  in  the  section,  and  from  thence 
passes  Into  the  cylinder  through  a  sliding-valve,  alternately  to 
each  side  of  the  piston  P,  as  is  represented  by  the  direction  of 
the  arrows,  the  sliding-valve  being  moved  by  the  rod  V,  commu- 
nicating with  an  "  eccentric  "  apparatus  attached  to  the  axis  of 
the  fly-wheel.  The  direction  of  the  current  of  steam  to  the 
valve-box  is  represented  by  the  arrow  at  I,  and  its  passage  out- 
ward from  the  cylinder,  after  it  has  moved  the  piston,  is  seen  at 
0.  In  this  engine  there  is  no  working-beam,  as  in  Watt's 
engine,  Fig.  109,  but  the  motion  is  communicated  from  the  pis- 
ton-rod to  a  crank  connected  wilh  the  fly-wheel,  which,  turning 
the  wheel,  will  move  all  machinery  connected  either  with  the 
axle  or  the  circumference  of  that  wheel. 

Fig.  115  represents  the  Locomotive  Steam-engine  in  one  of 
its  most  perfect  forms,  as  used  on  railways  at  the  present  day. 

772.  OPTICS. —  Optics  is  the  science  which 
What  is  Optics?  c  ,.  -         f       f  ,     .     .  . 

treats  of  light,  of  colors,  and  of  vision. 

How  are  all  sub-        773.  The  science  of  Optics  divides  all  sub 
stances   consid-    stances  into  the  following  classes :  namely 
luminous,  transparent,  and  translucent;  re- 
flecting, refracting,  and  opaque. 

What  are  lumi-  ^^-  Luminous  bodies  are  those  which 
nous  bodies  f  8^ne  ty  their  own  light — such  as  the  sun, 
the  stars,  a  burning  lamp,  or  a  fire. 

There  are  in  addition  to  the  above  quite  a  number  of  truly 
luminous  bodies,  but  of  slight  importance  so  far  as  their  light  is 
concerned,  because  of  its  faintness.  Such  are  the  fire-fly,  glow- 
worm, decaying  moist  wood,  and  to  these  may  be  added  certaic 
mineral  substances,  such  as  the  diamond,  \\  hich  are  luminous  for 
a  time  after  exposure  to  bright  sunlight 


OPTICS.  211 

What  are  trans-       775.  Transparent     substances    are    those 


sub-          which    allow    light   to   pass    through   them 
freely,  so  that  objects  can  be  distinctly  seen 
through  them;  as  glass,  water,  air,  &c.* 

776.  Translucent  bodies  are  those  which 
lucenlTodllsT'  Permifc  a  portion   of  light  to  pass  through 
them,  but  render  the  object  behind  them  in- 
distinct ;  as  horn,  oiled  paper,  colored  glass,  &c. 

What  are  re-  ^7.  Reflecting  substances  are  those  which 
fleeting  sub-  do  not  permit  light  to  pass  through  them; 
but  throw  it  off  in  a  direction  more  or  less 
oblique,  according  as  it  falls  on  the  reflecting  surface  ;  as 
polished  steel,  looking-glasses,  polished  metal,  &c. 

778.  Refracting  substances  are  those  which 
What    are  re-  ,      ,.   ,       ,,          .  . 

fracting  sub-  turn  tne  "&"*  "  fr°m  lts  course  m  its  passage 
stances  ?  through  them  ;  and  opaque  substances  are 

those  which  permit  nc  light  to  pass  through  them,  as  met- 
als, wood,  &c. 
What  is  light?        779.  It  is  not  known  what  light  is.     Sir 

What   are  the    jsaac  Newton    supposed    it    to    consist   of 

two  tncories  re-  *  A  . 

spelling  the  na-    exceedingly    small    particles,    moving    from 

tur  t  oj  ught?  luminous  bodies;  others  think  that  it  con- 
sists of  the  undulations  of  an  elastic  medium,  which  fills 
all  space,  f  These  undulations  (as  is  supposed)  produce  the 

*  No  substance  that  exists  on  our  earth  is  perfectly  transparent,  and  light 
must,  therefore,  necessarily  be  impaired  in  its  passage  through  all  transpa* 
rent  media,  and  the  diminution  it  suffers  will  vary  as  the  medium  is  more 
or  less  transparent,  and  as  the  passage  it  makes  is  of  greater  or  less  langth. 
The  exact  ratio  in  which  light  is  diminished  has  not  yet  been  determined  ; 
it  is,  however,  an  established  fact,  that  even  those  bodies  which  approach 
most  nearly  to  perfect  transparency  become  opaque  when  their  thickness  ia 
Donsiderably  increased. 

+  These  two  theories  of  light  are  called  respectively  the  corpuscular  and 
the  undulatnry  theory.  By  the  former  the  reflection  of  light  is  supposed  to 
lake  place  in  the  same  manner  as  the  reflection  of  solid  elastic  bodies,  as 
has  been  explained  under  the  head  of  Mechanics  [see  No.  1(55,  pnge  49] 
By  the  Litter  the  propagation  of  light  takes  place  from  every  luminous 
ouiut.by  meacs  of  the  undulatnry  movements  of  the  ether.  On  this  hypotb- 

9* 


212  NATURAL   PHILOSOPHY. 

sensafron  of  light  to  the  eje,  in  the  same  manner  as  th« 
vibrations  of  the  air  produce  the  sensation  of  sound  to  the 
ear.  The  opinions  of  philosophers  at  the  present  day  art 
inclining  to  the  undulatory  theory.  (See  par.  1476.) 
What  is  a  ray  780.  A  ray  of  light  is  a  single  line  of 
of  hght  /  \ig\\t  proceeding  from  a  luminous  body. 

781.  Rays  of  light  are  said   to  diverge 
When  are  rays        •,        , , 
said  to  diverge?    wnen  theJ  separate  more  Fig.  ne. 

widely   as  they  proceed 
from  a  luminous  body. 

Fig.   116  represents   the 
^rp  a        ig.     rayg  Q£  jjgjjj.  diverging  as  they  proceed  from  the 
luminous  body,  from  F  to  D. 

782.  It  will  be  seen  by  this  figure  that,  as  light  is  projected  In 
every  direction,  its  intensity  must  decrease  with  the  distance,  and 
this  decrease  is  determined  by  a  fixed  law.  The  light  received  upon 
any  surface  decreases  as  the  square  of  the  distance  increases. 
Thus,  if  a  portion  of  light  fall  on  a  surface  at  the  distance  of  two 
feet  from  any  luaiinary,  a  surface  twice  that  distance  will  receive 
only  one-fourth  as  much  light;  at  three  times  that  distance,  one- 
ninth  ;  at  four  times  the  distance,  one-sixteenth,  £c.  Hence  a  per- 
son can  see  to  read  at  a  short  distance  from  a  single  lamp  much 
better  than  at  twice  the  same  distance  with  two  lamps,  &c. 

When  are  rays  783.  Rays  of  light  are  said  to  converge 
of  Hght  said  to  ,  .  ,  ,  ,  mi 

converge?  when  they  approach  each  other.     The  point 

esis,  the  waves  of  light  follow  the  general  laws  of  the  reflection  of  all 
elastic  fluids ,  and,  accordingly,  every  wave  from  every  point,  when  it  im- 
pinges on  any  resisting  object  so  as  to  be  reflected,  forms  a  new  wave  in  its 
course  back,  having  its  centre  as  much  on  the  other  side  of  the  obstacle  as 
the  centre  of  the  original  wave  was  on  this  side.  In  the  case  of  light  the 
centre  of  the  original  wave  is,  obviously,  the  luminous  point.  There  is  a 
remarkable  similarity,  therefore,  between  the  reflection  of  light,  pud  echo 
or  the  reflection  of  sound.  It  has  been  shown,  under  the  head  of  Acc-usMc^, 
that  when  two  waves  meet  under  certain  circumstances,  the  elevation  oi 
one  wave  exactly  filling  up  the  depression  of  another  wave,  produces  who,1 
is?  called  the  acoustic  paradox,  namely,  two  sounds  producing  silt-nee.  It  wiT 
readily  be  seen  that  the  same  undulatory  movements  in  Optics  will  produce 
the  same  analogous  effect  ;  or,  in  other  words,  that  two  ray*  of  lizht  n>,i^ 
produce  darkness  ;  and  this  may,  with  equal  propriety,  be  termed  the  optical 
paradox.  T3ut  a  clear  understa  tiding  of  the  principles  involved  in  what 
is  called  respectively  the  hydrostatic,  pneumatic,  acoust*''  »nd  optical  para 
dox,  shows  that  there  is  no  paradox  at  all,  but  that  each  ia  the  necessarj 
result  of  oertai-  fixed  and  determinate  laws 


OPTICS. 

:tt  which  converging  rays  meet  is  called  F;K  n?. 

the  focus. 

Fig.  117  represents  con- 
Erplain    fig.     verging    rays    of  light>   of 

which  the  point  F  is  the  focus. 

What  is  abeam        784.  A   beam   of     Fig'118- 

of  light ;  light  consists  of  many 

rays  running  in  parallel  lines. 

Explain    Fig.     Fig>  118  represents  a  beam  of  light. 

785.  A  pencil  of  rays  is  a  collection  of 
What  ts  a  pen-     , .          .  .    r  r  0         ,  T . 

ci/  of  rays?        diverging  or  converging  rays.    [&*    F.#», 

116  and  117.] 

786.  Light  proceeding    from  a   luminous 
In   what    dtrec-   .     ,     .  .  ,  .  . 

tion,  and  with  body  is  projected  forward  m  straight  lines  in 

what    rapidity,  every  possible    direction.     It  moves  with   a 
does  light  mo  vet          . ,.       ,        1-1,  i        111 

rapidity  but  little  short  of  two  hundred  thou  • 

sand  miles  in  a  second  of  time. 

787.  Every  point  of  a  luminous  body  ia 

From  what  part  «  ,  .  ,    , .   ,  , . 

of  a  luminous    a  Centre5  trom  which  light  radiates  in  every 

body  does  ligkt  direction.  Rays  of  light  proceeding  from 
different  bodies  cross  each  other  witliO'U 
interfering.  The  rays  of  light  which  issue  from  terrestrial 
bodies  continually  diverge,  until  they  meet  with  a  refract- 
ing substance ,  out  the  rays  of  the  sun  diverge  so  little,  on 
account  of  the  immense  distance  of  that  luminary,  that  they 
are  considered  parallel. 

What  is  a  788.   A  shadow  is  the  darkness  produced 

shadow  ?  ky  the  intervention  of  an  opaque  body,  which 

prevents  the  rays  of  light  from  reaching  an  object  behind 
the  opaque  body. 

Why  are  shad-       789'    Sliaclows  are  of  different  degrees  of 
'  oic»  of  different  darkness,  because  the  light  from  other  luml- 


214  NATURAL    PHILOSOPHY. 

degrees  oj dark-  nous  bodies  reaches  the  spot  where  the 
shadow  is  formed.  Thus,  if  a  shadow  be 
formed  when  two  candles  are  burning  in  a  room,  that 
shadow  will  be  both  deeper  and  darker  if  one  of  the  can  • 
dies  be  extinguished.  The  darkness  of  a  shadow  is  propor- 
tioned to  the  intensity  of  the  light,  when  the  shadow  is 
produced  by  the  interruption  of  the  rays  froui  a  single 
luminous  body. 

What  produces  79°-  As  the  degree  of  Hght  and  darkness 
the  darkest  can  be  estimated  only  by  comparison,  the 
strongest  light  will  appear  to  produce  the 
deepest  shadow.  Hence,  a  total  eclipse  of  the  sun  occa- 
sions a  more  sensible  darkness  than  midnight,  because  it  is 
immediately  contrasted  with  the  strong  light  of  day.  Hence 
also,  by  causing  the  shadow  of  a  single  object  to  be  thrown 
on  a  surface.  —  as,  for  instance,  the  wall, — from  two  or  mor* 
lights,  we  can  tell  which  is  the  brightest  light,  because  it 
will  cause  the  darkest  shadow. 

791.  When  a  luminous  body  is  larger  than 
What    is    the  ,     ,        ,         ,     ,  f  ,, 

thape   of  the    an  opaque  body,  the  shadow  of  the  opaque 

shadow  of  an    body  will   gradually  diminish  in  size  till  it 
('ue  terminates   in    a   point.     The    form    of   the 

shadow  of  a  spherical  body  will  be  that  of  a  cone. 

Fig.  119.     A  repre- 
Kxplain    Fig.     gents  th(J   ^  ^  B 

the  moon.  The  sun 
being  much  larger  than  the  moon, 
causes  it  to  cast  a  converging  shadow, 
which  terminates  at  E. 

792.  When  the  luminous  body  is  smaller  than  the 
opaque  body,  the  shadow  of  the  opaque  body  will  gradually 
increase  in  size  with  the  distance,  without  limit. 


OPTICS. 


9,15 


IK    Fig.  120  the  shadow  *«•  12°- 

of  the-  object  A  increases 
ui  size  at  the  different  dis- 
tances B,  C,  D,  E;  or,  in 
other  words,  it  constantly 
diverges. 

793.  When  several  luminous  bodies  shine  upon  the  same 
object,  each  one  will  produce  a  shadow. 

What  is  it  the  Fig.  121  represents  a  ball  A,  illuminated  by 
object  of  Fig.  the  three  can- 
dles B,  C,  .and 
D.  The  light  B  produces  the 
shadow  3,  the  light  C  the  shadow 
c,  and  the  light  D  the  shadow  d  ; 
but,  as  the  light  from  each  of  th^ 
candles  shines  upon  all  the  shad- 
ows except  its  own,  the  shadows 
will  be  faint. 


Fig.  121. 


What  becomes  of 
the   light   which 
falls  on  an 
upaque  object? 


When  is  light 
said  to  be  re- 
flected? 


794.  When  raya  of  light  fall  upon  an 
opaque  body,  part  o.^  them  are  absorbed,  and 
part  are  reflected. 

Light  is  said  to  be  reflected  when  it  is 
thrown  off  from  the  body  on  which  it  falls ; 
and  it  is  reflected  in  the  largest  quantities 
from  the  most  highly  polished  surfaces.  Thus,  although 
most  substances  reflect  it  in  a  degree,  polished  metals,  look- 
ing-glasses, or  mirrors,  &c.,  reflect  it  "in  so  perfect  a  man- 
ner as  to  convey  to  our  eyes,  when  situated  in  a  proper 
position  to  receive  them,  perfect  images  of  whatever  objects 
shine  on  them,  either  by  their  own  or  by  borrowed  lio;ht. 

795.  That  part  of  the  science  of  Optics 
which   relates    to   reflected   light    is   called 
Outuptricx. 


What  is  Catop- 


NATURAL    PHILOSOPHY. 


Wnat  is  the  fun-          T96>  The  laws  of  r^cted  light  are  the 
damenfal  law  of      same  as  those  of  reflected  motion.     Thus, 
atop  no,  when  light    falls    perpendicularly    on    an 

opaque  body,  it  is  reflected  back  in  the  same  line  towards 
the  point  whence  it  proceeded.  If  it  fall  obliquely,  it  will 
ta  reflected  obliquely  in  the  opposite  direction ;  and  in  all 
cases  the  angle  of  incidence  will  be  equal  to  the  angle  of 
reflection.  This  is  the  fundamental  law  of  Catoptrics,  or 
reflected  light. 

797.  The  angles  of  incidence  and  reflection  have  already  beer 
described  under  the  head  of  Mechanics  [see  explanation  of 
/<%•.  10,  No.  162] ;  but,  as  all  the  phenomena  of  reflected  light 
depend  upon  the  law  stated  above,  and  a  clear  idea  of  these 
angles  is  necessary  in  order  to  understand  the  law,  it  is  deemed 
expedient  to  repeat  in  this  connection  the  explanation  already 
given. 

An  incident  ray  is  a  ray  proceeding  to  or^  falling  on  any  sur- 
face ;  and  a  reflected  ray  is  the  ray  which  proceeds  frorr  any 
reflecting  surface. 

Fig.  122  is  designed  to  show 
the  angles  of  incidence  and  of 
reflection.  In  this  figure,  M 
A  M  is  a  mirror,  or  reflecting  surface.  P  is 
a  line  perpendicular  to  the  surface.  I  A  rep- 
resents an  incident  ray,  falling  on  the  mirror 
in  such  a  manner  as  to  form,  with  the  perpen- 
dicular P,  the  angle  I  A  P.  This  is  called 
the  angle  of  incidence.  The  line  R  A  is  to 
be  drawn  on  the  othor  side  of  P  A  in  such  a  manner  as  to  have 
the  same  inclination  with  P  A  as  I  A  has  :  that  is,  the  angle 
K  A  P  is  equal  to  I  A  P.  The  line  R  A  will  then  show  the 
course  of  the  reflected  ray ;  and  the  angle  RAP  will  be 
fche  angle  of  reflection. 

From  whatever  surface  a  ray  of  light  is  reflected, — whether  it 
be  a  plain  surface,  a  convex  surface,  or  a  concave  surface, — this 


Explain   Fig. 
122. 


Fig.  122. 


OPTICS.  217 

law  invariably  prevails  ;  so  that,  if  we  notice  the  inclination  of 
any  incident  raj,  and  the  situation  of  the  perpendicular  to  the 
surface  on  which  it  falls,  we  can  always  determine  in  what  man- 
ner or  to  what  point  it  will  be  reflected.  This  law  explains  thg 
reason  why,  when  we  are  standing  on  one  side  of  a  mirror,  we 
can  see  the  reflection  of  objects  on  the  opposite  side  of  the  room, 
but  not  those  on  the  same  side  on  which  we  are  standing.  It  also 
explains  the  reason  why  a  person  can  see  his  whole  figure  in  a 
mirror  not  more  than  half  of  his  height.  It  also  accounts  for 
all  the  apparent  peculiarities  of  the  reflection  of  the  different 
kinds  of  mirrors. 

How  are  lu-  798.  Opaque  bodies  are  seen  only  by  re- 
minous  and  flected  light.  Luminous  bodies  are  seen  by 

opaque  bodies  '  . 

respectively       the  rays  of  light  which   they  send  directly  to 

seen  ?  Qur 


What  effect  799.  All  bodies  absorb  a  portion  of  the  light 
ontlwinten-  which  they  receive  ;  therefore  the  intensity  of 
sity  of  light?  light  is  diminished  every  time  that  it  is  reflected. 
What  does  800.  Every  portion  of  a  reflecting  surface 
^a  reflecting  reflects  an  entire  image  of  the  luminous  body 

surface  reflect  ?  shining  upon  it. 

T,r,     ,  Whon   the   sun    or   the   moon   shines   upon   a 

H-  liy  do  we 

not  see  many    sheet  of  water,  every  portion  of  the  surface  reflects 

images  of  the  an  entire  image  of  the  luminary;  but,  as  the  image 
same  thing  n  . 

reflected  by  a     can  "e    seen    onv    "J  reflected  rays,    and  as  the 
reflecting  sur-  angle  of  reflection  is  always  equal  to  the  angle  of 
incidence,  the  image  from  any  point  can  be  ~,een 
only  in  the  reflected  ray  prolonged. 

Why  do  objects  801.  Objects  seen  by  moonlight  appear  faiutc; 
tppear  fainter  than  when  seen  by  daylight,  because  the  light  by 
'  which  they  are  seen  has  been  twice  reflected  ;  for, 
the  moon  is  not  a  luminous  body,  but  its  light  is  caused  by  thu 
Hun  shining  upon  it.  This  light,  reflected  from  the  moon  u.nd 
fulling  upon  any  object,  is  again  reflected  by  tba*  object.  If 


218  NATURAL    PHILOSOUiY. 

Buffers,  therefore,  two  reflections  ;  and  since  a  portion  is  absorbed 
by  each  surface  that  reflects  it,  the  light  must  be  proportion*- 
illy  fainter.  In  traversing  the  atmosphere,  also,  the  rays  Loth 
of  the  sun  and  moon,  suffer  diminution  ;  for,  although  pure  air 
is  a  transparent  medium,  which  transmits  the  rays  of  light 
freely,  it  is  generally  surcharged  with  vapors  and  exhalations, 
by  which  some  portion  of  light  is  absorbed. 

„„  802.  All  objects  are  seen  by  means  of  the 

When  is  an  .  J  J 

object  invisi-  rays  of  light  emanating  or  reflected  from  them  ; 
ble-  and  therefore,  when  no  light  falls  upon  an 

opaque  body,  it  is  invisible. 

This  is  the  reason  why  none  but  luminous  bodies  can  be 
seen  in  the  dark.  For  the  same  reason,  objects  in  the  shade  or 
in  a  darkened  room  appear  indistinct,  while  those  which  are 
exposed  to  a  strong  light  can  be  clearly  seen.  We  see  the 
things  around  us,  when  the  sun  does  not  shine  directly  upon  them, 
solely  by  means  of  reflected  light.  Everything  on  which  it 
shines  directly  reflects  a  portion  of  its  rays  in  all  possible  direc- 
tions, and  it  is  by  means  of  this  reflected  light  that  we  are 
enabled  to  see  the  objects  around  us  in  the  day-time  which  are 
not  in  the  direct  rays  of  the  sun.  It  may  here  also  be  remarked 
that  it  is  entirely  owing  to  the  reflection  of  the  atmosphere  that 
the  heavens  appear  bright  in  the  day-time.  If  the  atmosphere 
had  no  reflective  power,  only  that  part  would  be  luminous  in 
which  the  sun  is  placed  ;  and,  on  turning  our  back  to  the  sun,  the 
whole  heavens  would  appear  as  dark  as  in  the  night  ;  we  should 
have  no  twilight,  but  a  sudden  transition  from  the  brightest 
sunsb?ne  to  darkness  immediately  upon  the  setting  of  the  sun. 

803.    When  rays  of  light,  proceeding  from 
liht  enter  an7  object,  enter  a  small  aperture,  they  cross 


enter 
a  small  aper-  one  another,  and  form  an  inverted  image  of  the 

object.     This  is  a  necessary  consequence  of  the 
law  that  light  always  moves  in  straight  lines. 
K.rj>/am          ^04.  Fig.  123  represents   the  rays  from  an  object 
ttV.  123     a  c,  entering  an  aperture.     The  ray  from  a  passes 


OPTICS. 

down  through   the  aperture   to  d,  and  the  ray  from    c 
up  to  f>,  and  thus  these  rays,  crossing  at  the  aper-        Fi«  Jv!3- 
ture,  form  an  inverted  image  on   the  wall.     The 
room  in  which  this  experiment  is  made  should  be 
darkened,  and  no  light  permitted  to  enter,  except- 
ing through  the   aperture.     It  then  becomes  a 
camera  obscura. 

805.  These  words  signify  a  darkened  chamber.  In  the  future  de 
Bcription  which  will  be  given  of  the  eye,  it  will  be  setn  that  the 
camera  obscura  is  constructed  on  the  same  principle  as  the  eye.  If  a 
convex  lens  be  placed  in  the  aperture,  an  inverted  picture,  not  only 
of  a  single  object,  but  of  the  entire  landscape,  will  be  found  on  the 
wall.  A  portable  camera  obscura  is  made  by  admitting  the  light 
into  a  box  of  any  gize,  through  a  convex  lens,  which  throws  the 
image  upon  an  inclined  mirror,  from  whence  it  is  reflected  upwards 
to  a  plate  of  ground  glass.  In  this  manner  a  beautiful  but  dimin- 
ished image  of  the  landscape,  or. of  any  group  of  objects,  is  present- 
ed on  the  plate  in  an  erect  position. 

What  is  the        806.    The  angle  of  vision  is  the  angle  formed 

angle  of  at  the  eye  by  two  lines  drawn  from  opposite 
vision?  f  J 

parts  ot  an  object. 

What  is  the  807.  The  angle  C,  in  Fig.  124,  repiesents  the 
"figures  124  ano^e  °^  visi°n-  The  line  A  C,  proceeding  from 
and  125  ?  one  extremity  of  the  object,  meets  the  line  B  C 
from  the  opposite  extrem-  Fig.  124. 

ity,  and  forms  an  angle  G 
at  the  eye  ;  —  this  is  the 
angle  of  vision. 

808.  Fig.  125  represents 
the   different  angles  made 
by  the  same  object  at  dif- 
ferent distances.     From  an  inspection  of 
the  figure,  it  is  evident  that  the  nearer 
the  object  is  to  the  eye,  the  wider  must    c 
be  the  opening  of  the  lines  to  admit  the 
extremities  of  the  object,  and,  consequent-  E        B 

ly,  the  larger  the  angle  under  which  it  is  seen  ;  and,  on  the  con 
trary,  that  objects  at  a  distance  will  form  small  angles  of  vision 
Thug,  in  this  figure,  the  three  crosses  F  G,  D  E,  and  A  B.  are 


NATURAL   PHILOSOPHY. 

all  of  the  same  size  ;  but  A  B,  being  the  most  distant,  subtesdr 
the  smallest  angle  A  C  B,  while  D  E  and  F  G,  being  nearer  to 
the  eye,  situated  at  C,  form  respectively  the  larger  angles  DOE 
and  FOG. 

809.  The  apparent  size  of  an  object  depends  upon 
On  what  docs   ,       .        «  .,  ,       ,.     .  . 

the  apparent  tne  size  °*  tne  angle  °*  vision.  But  we  are  accus- 
size  of  an  ob-  tomed  to  correct,  by  experience,  the  fallacy  of  ap- 
yect  depend  ?  pearances  j  ari&t  therefore,  since  we  know  that  real 
objects  do  not  vary  in  size,  but  that  the  angles  under  which  we 
see  them  do  vary  with  the  distance,  we  are  not  deceived  by  the 
variations  in  the  appearance  of  objects. 

Thus,  a  house  at  a  distance  appears  absolutely  smaller  than  the 
window  through  which  we  look  at  it  ;  otherwise  we  could  not  see 
it  through  the  window  ;  but  our  knowledge  of  the  real  size  of  the 
house  prevents  our  alluding  to  its  apparent  magnitude.  In  Fig.  124 
in  will  be  seen  that  the  several  crosses,  A  B,  D  E,  F  G,  and  II  I, 
although  very  different  in  size,  on  account  of  their  different  distances, 
subtend  the  same  angle  A  C  B  ;  they,  therefore,  all  appear  to  the 
eye  to  be  of  the  same  size,  while,  in  Fig.  125,  the  three  objects  A  B, 
D  E,  and  F  G,  although  of  the  same  absolute  size,  are  seen  at  a  dif- 
ferent angle  of  vision,  and  they,  therefore,  will  seem  of  different 
sizes,  appearing  larger  as  they  approach  the  eye. 

It  is  to  a  correct  observance  of  the  angle  of  vision  that  the  art  of 
perspective  drawing  is  indebted  for  its  accuracy. 

When  is  an        810.  When  an  object,  at  any  distance,  does 
of  not  subtend  an  angle  of  more  than  two  seconds 


its  distance  ?  Of  a  degree,  it  is  invisible. 

At  the  distance  of  four  miles  a  man  of  common  stature 
will  thus  become  invisible,  because  his  height  at  that  distance 
will  not  subtend  an  angle  of  two  seconds  of  a  degree.  The  size 
of  the  apparent  diameter  of  the  heavenly  bodies  is  generally 
stated  by  the  angle  which  they  subtend. 

Wh  •  811.  When  the  velocity  of  a  moving  body 

tion  imper-  does  not  exceed  twenty  degrees  in  an  hour,  its 
teptible  /  motion  is  imperceptible  to  the  eye. 

It  is  for  this  reason  that  the  motion  of  the  heavenly 
todies  is  invisible,  notwithstanding  their  immense  velocity. 

812.  The  real  velocity  of  a  body  in  motion  round  a  point  de- 
pends on  the  spuee  comprehended  in  a  degree.  The  more  dis- 


OPTICS.  C^i 

tunj  the  moving  body  from  the  centre,  or,  in  other  words,  the 
larger  the  circle  which  it  has  to  describe,  the  larger  will  be  the 
degree. 

813.  In  Fig.  126,  if  the  man  at  A,  and  the  **•  126- 
man  at  B,  both  start  together,  it  is  manifest 
that  A  must  move  more  rapidly  than  B,  to 
arrive  at  C  at  the  same  time  that  B  reaches 
D,  because  the  arc  A  C  is  the  arc  of  a  larger 
circle  than  the  arc  B  D.  But  to  the  eye  at  E 
the  velocity  of  both  appears  to  be  the  same,  C  ^  B 

because  both  are  seen  under  the  same  angle  of  vision. 

uri  814.  A  mirror  is  a  smooth  and  polished  sur- 

Wfiat  are 

mirrors,  and   face,  that  forms  images  by  the  reflection  of  the 


^  ^»nk  Mirrors  (or  looking-glasses)  are 
made  of  glass,  with  the  back  covered  with  an 
amalgam,  or  mixture  of  mercury  and  tin  foil.  It  is  the 
smooth  and  bright  surface  of  the  mercury  that  reflects  the 
rays,  the  glass  acting  only  as  a  transparent  case,  or  cover- 
ing, through  which  the  rays  find  an  easy  passage.  Some 
of  the  rays  are  absorbed  in  their  passage  through  the  glass, 
because  the  purest  glass  is  not  free  from  imperfections.  For 
this  reason,  the  best  mirrors  are  made  of  an  alloy  of  copper 
and  tin,  called  speculum  metal. 

What  are  the  815>  There  are  three  kinds  of  mirrors, 
different  kinds  namely,  the  plain,  the  concave,  and  the  con- 
of  mirrors  f 

vex  mirror. 

Plain  mirrors  are  those  which  have  a  flat  surface,  such 
as  a  common  looking-glass  ;  and  they  neither  magnify  nor 
diminish  the  image  of  objects  reflected  from  them. 

816.  The  reflection  from  plain  mirrors  is  always 
By  what  law 
are  objects  re-    obedient  to  the  law  that  the  angles  of  incidence  and 

fleeted  from  a   reflection  are  equal.     For  this  reason,  no  person 
'  can  see  another  in  a  looking-glass,  if  the  other  can- 
not see  him  in  return. 


BTATUBAL   PHILOSOPHY. 

How  do  looUng-  817-  Looking-glasses  or  plain  mirrors  cause 
glasses  make  all  everything  to  appear  reversed.  Standing  before 
Ejects  appear?  &  iooking-glass,  if  a  person  holds  up  his  left 
hand  it  will  appear  in  the  glass  to  be  the  right. 

818.  A  looking-glass,  to  reflect  the  whole  person,  needs  be  but  half 
of  the  length  of  the  person. 

819.  When  two  plain  mirroTS  stand  opposite  to  each  other,  the 
reflections  of  the  one  are  cast  upon  the  other,  and  to  a  person  be- 
tween them  they  present  a  long-concinued  vista. 

820.  When  two  reflecting  surfaces  are  inclined  at  an  angle,  the 
reflected  objects  appear  to  have  a  common  centre  to  an  eye  viewing 
themv  obliquely.     It  is  on  this  principle  that   the  kaleidoscope  is 
constructed. 

What  is  a  821.  The  Kaleidoscope  consists  of  two  reflecting 

Kaleidoscope?  surfaces,  or  pieces  of  looking-glass,  inclined  to 
each  other  at  an  angle 'of  sixty  degrees,  and  placed  between 
the  eye  and  the  objects  intended  to  form  the  picture. 

The  two  plates  are  enclosed  in  a  tin  or  paper  tube,  and  the 
objects,  consisting  of  pieces  of  colored  glass,  beads,  or  other 
highly-colored  fragments,  are  loosely  confined  between  two  cir- 
cular pieces  of  common  glass,  the  outer  one  of  which  is  slightly 
ground,  to  make  the  light  uniform.  On  looking  down  the  tube 
through  a  small  aperture,  and  where  the  ends  of  the  glass  plates 
nearly  meet,  a  beautiful  figure  will  be  seen,  having  six  angles, 
the  reflectors  being  inclined  the  sixth  part  of  a  circle.  If  in- 
clined the  twelfth  part  or  twentieth  part  of  a  circle,  twelve  or 
twenty  angles  will  be  seen.  By  turning  the  tube  so  as  to  alter 
the  position  of  the  colored  fragments  within,  these  beautiful  forma 
will  be  changed  ;  and  in  this  manner  an  almost  infinite  variety 
of  patterns  may  be  produced. 

The  word  Kaleidoscope  is  derived  from  the  Greek  language,  and 
means  "  the  sight  of  a  beautiful  form."  The  instrument  was  in- 
dented by  Dr.  Brewster,  of  Edinburgh,  a  few  years  ago. 

822.  A  convex  mirror  is  a  portion  of  the  external  sur- 
face of  a  sphere.  Convex  mirrors  have  therefore  a  convex 
surface. 

823    A  concave  mirror  is  a  portion  of  the  inner  surface 


01TICS. 


The  outer  part  of  M  N  is  a 

Fig.  127. 


of  a  hollow  sphere.  Concave  mirrors  have  therefore  a  con- 
cave surface. 

Exj-ilain        824.  In  Fig.  127,  M  N   represents  both  a  convex 
rig.  127.  and  a  concave  mirror.     They  are  both  a  portion  of  a 
sphere  of  which  0  is  the  centre, 
convex,  and  the  inner  part  is 
a  concave  mirror.     Let  A  B, 
C    D,   E    F,    represent    rays 
falling  on  the  convex  mirror 
M  N.     As  the  three  rays  are 
parallel,  they  would  all  be  per- 
pendicular to  a  plane  or  flat 
mirror ;  but   no  ray  can  fall 
perpendicularly  on  a  concave 
or  convex  mirror  which  is  not 

directed  tmvards  the  centre  of  the  sphere  of  which  the  mirror  is 
a,  portion.  For  this  reason,  the  ray  C  D  is  perpendicular  to  the 
mirror,  while  the  other  rays,  A  B  and  E  F,  fall  obliquely  upon 
it.  The  middle  ray  therefore,  falling  perpendicularly  on  the 
mirror,  will  be  reflected  back  in  the  same  line,  while  the  two 
other  rays,  falling  obliquely,  will  be  reflected  obliquely  ;  namely, 
the  ray  A  B  will  be  reflected  to  G,  and  the  ray  E  F  to  H,  and 
the  angles  of  incidence  A  B  P  and  EFT  will  be  equal  to  the 
angles  of  reflection  P  B  G  and  T  F  H  ;  and,  since  we  see  objects 
in  the  direction  of  the  reflected  rays,  we  shall  see  the  image  at 
L,  which  is  the  point  at  which  the  reflected  rays,  if  continued 
through  the  mirror,  would  unite  and  form  the  image.  This  point 
is  equally  distant  from  the  surface  and  the  centre  of  the  sphere 
and  is  called  the  imaginary  focus  of  the  mirror.  It  is  called  the 
imaginary  focus,  because  the  rays  do  not  really  unite  at  that 
point,  but  only  appear  to  do  so ;  for  the  rays  do  not  pass  through 
the  mirror,  since  they  are  reflected  by  it. 

825.  The  image  of  an  object  reflected  from  a  convex 
oairror  is  smaller  than  the  object 


224: 


NATUKAL   PHILOSOPHY. 


What  is  the  ^®'  This  is  owing  to  the  divergence  of  the  re- 
ooject  of  fleeted  rays.  A  convex  mirror  converts,  try  reflec- 

TTV/y    "1  9Q  9 

ff'  tion,  parallel  rays   into   divergent  rays;   rays  that 

fall  upon  the  mirror  divergent  are  rendered  still  more  diver- 
gent by  reflection,  and  convergent  rays  are  reflected  either 
parallel,  or  less  con-  Oig.  128. 

vergent.  If,  then,  an 
object,  A  B,  be  placed 
before  any  part  of  a 
convex  mirror,  the 
two  rays  A  and  B, 
proceeding  from  the 
extremities,  falling 
convergent  on  the 
mirror,  will  be  re- 
flected less  convergent,  and  will  not  come  to  a  focus  until  they 
arrive  at  C  ;  then  an  eye  placed  in  the  direction  of  the  reflected 
rays  will  see  the  image  formed  in  (or  rather  behind)  the  mirror 
at  a  b  ;  and,  as  the  image  is  seen  under  a  smaller  angle  than  the 
object,  it  will  appear  smaller  than  the  object. 

What  is  the  $27.  The  true  focus  of  a  concave  mirror  is 
true  focus  of  a  point  equally  distant  from  the  centre  and  the 
'mirror?  surface  of  the  sphere  of  which  the  mirror  is  a 

portion. 

When  will  828.  When  an  object  is  further  from  the  con- 
ike  image  re-  cave  surface  mirror  than  its  focus,  the  image  will  be 
aconcaveTe  inverted;  but  when  the  object  is  between  the 
upright,  and  mirror  and  its  focus,  the  image  will  be  upright, 
'ed  T  "  an^  Srow  larger  *n  proportion  as  the  object  is 

placed  nearer  to  the  focus. 

What  pe-  829.  Concave  mirrors  have  the  peculiar  prop- 
culiar  prop-  erty  of  forming  images  in  the  air.  The  mirror 
^oncar^mir-  an(*  tne  ODJect  being  concealed  behind  a  screen, 
rnrs?  or  a  wall,  and  the  object  being  strongly  illumi- 


OPTIOJ. 


Dated,  the  ra^  from  the  object  fail  upon  the  minor,  and  are 
reflected  by  it  through  an  opening  in  the  screen  or  wall,  forming 
an  image  in  the  air. 

Showmen  have  availed  themselves  of  this  property  of  concave 
mirrors,  in  producing  the  appearance  of  apparitions,  which  have 
terrified  the  young  and  the  ignorant.  These  images  have  been  pre- 
sented with  great  distinctness  and  beauty,  by  raising  a  fine  trans- 
parent cloud  of  blue  smoke,  by  means  of  a  chafing-dish,  around  the 
focus  of  a  large  concave  mirror. 

When  is  the  830.  The  image  reflected  by  a  concave 

image  from  a      mirror  js  larger  than  the  object  when  the 

concave  mirror  °  .T 

larger  than  the   object  is  placed  between  the  mirror  and  its 

<**"*'  focus. 


Fig.  129. 


What  is  the  de-  831-  This  is  owing  to  the  convergent  prop- 
iign  of  Fig.  erty  of  the  concave  mirror.  If  the  object  A 
B  be  placed  between  the  concave  mirror  and  ifr» 
focus  /,  the  rays 
A  and  B  from  its 
extremities  will 
fall  divergent  on 
the  mirror,  and, 
on  being  reflect- 
ed, become  less 
divergent,  as  if 
they  proceeded 
from  C.  To  an 
eye  placed  in  that  situation,  namely,  at  C,  the  image  will  appear 
magnified  behind  the  mirror,  at  a  I  since  it  is  seen  under  a 
larger  angle  than  the  object. 

832.  There  are  three  cases  to  be  considered  with  regard  to  the 
effects  of  concave  mirrors  : 

1.  When  the  object  is  placed  between  the  mirror  and  the  princi 
pal  focns. 

2.  When  it  is  situated  between  its  centre  of  concavity  and  that 
focus. 

3.  When  it  is  more  remote  than  the  centre  of  concavity. 

1st.     In  the  first  case,  the  rays  of  light  diverging  after  reflection 
but  in  a  less  degree  than  before  such  reflection  took  place,  the  iui 


2^6  NATURAL    PHILOSOPHY 

age  will  be  larger  than  the  object  and  appear  at  a  greater  01 
smaller  distance  from  the  surface  oi  the  mirror,  and  behind  it.  Tha 
Image  in  this  case  will  be  erect. 

2d.  When  the  object  is  between  the  principal  focus  and  the  cen- 
tre of  the  mirror,  the  apparent  image  will  be  in  front  of  the  mirror, 
and  beyond  the  centre,  appearing  very  distant  when  the  object  is 
at  or  just  beyond  the  focus,  and  advancing  towards  it  as  it  recedes 
towards  the  centre  of  concavity,  where,  as  will  be  stated,  the  im- 
age and  the  object  will  coincide.  During  the  retreat  of  the  object 
the  image  will  still  be  inverted,  because  the  rays  belonging  to  each 
visible  point  will  not  intersect  before  they  reach  the  eye.  But  In 
this  case  the  image  becomes  less  and  less  distinct,  at  the  same  time 
that  the  visual  angle  is  increasing;  so  that  at  the  centre,  or  rather 
a  little  before,  the  image  becomes  confused  and  imperfect,  because 
at  this  point  the  object  and  the  image  coincide. 

3d  In  the  cases  just  considered,  the  images  will  appear  inverted ; 
and  in  the  case  where  the  object  is  further  from  the  mirror  than  its 
centre  of  concavity,  the  image  will  be  inverted.  The  more  distant 
the  object  is  from  the  centre,  the  less  will  be  its  image  ;  but  the 
image  and  object  will  coincide  when  the  latter  is  stationed  exactly 
at  the  centre. 

833.  The  following  laws  flow  from  the  fundamental  law  of  Catop- 
trics,   namely,    that    the   angles   of   incidence   and   reflection   are 
always  equal.     In  estimating  these  angles,  it  must  be  recollected 
that  no  line  is  perpendicular  to  a  convex  or  concave  mirror,  which 
will  not,  when  sufficiently  prolonged,  pass  through  the  centre  cf  the 
sphere  of  which  the  mirror  is  a  portion.     The  truth  of  these  state- 
ments may  be  illustrated  by  simple  drawings  ;  always  recollecting, 
in  drawing  the  figures,  to  make  the  angles  of  incidence  and  reflec- 
tion equal.     The  whole  may  also  be  shown  by  the  simple  experi- 
ment of  placing  tho  flame  of  a  candle  in  various  positions  before 
both  convex  and  concave  mirrors.    [It  is  recommended  that  the  learner 
be  required  to  draw  a  figure  to  represent  each  of  these  laws.] 

834.  LAWS  OF  REFLECTION  FROM  CONVEX  MIRRORS. — (1.)  Par- 
allel rays  reflected  from  a  CONVEX  surface  are  made  to  diverge. 

(2.)  Diverging  rays  reflected  from  a  CONVEX  surface  are  made 
more  diverging. 

(3.)  When  converging  rays  tend  towards  the  focus  of  parallel 
rays,  they  will  become  parallel  when  reflected  from  a  CONVEX 
surface.* 

(4.)  When  converging  rays  tend  to  a  point  nearer  the  surface 

*  For  the  sake  of  distinction,  the  principal  focus  is  called  "  the   jbcus  0' 


OITIOS.  227 

than  the  focus,  they  will  converge  less  when  reflected  from  a 
CONVEX  surface. 

(5.)  If  converging  rays  tend  to  a  point  between  the  focus  and 
the  centre,  they  will  diverge  as  from  a  point  on  the  other  side 
of  the  centre,  further  from  it  than  the  point  towards  which  they 
converged. 

(6.)  If  converging  rays  tend  to  a  point  beyond  the  centre, 
they  will  diverge  as  from  a  point  on  the  contrary  side  of  the 
centre,  nearer  to  it  than  the  point  towards  which  they  con- 
verged. 

(7.)  If  converging  rays  tend  to  the  centre,  when  reflected 
they  will  proceed  in  a  direction  as  if  from  the  centre 

835.  LAWS  OF  REFLECTION  FROM  CONCAVE  MIRRORS. — 
(1.)  Parallel  rays  reflected  from  a  CONCAVE  Kirface  are  made 
converging.  [See  Note  to  No.  837.] 

(2.)  Converging  rays  falling  upon  a  CONCAVE  surface  are 
made  to  converge  more. 

(3.)  Diverging  rays  falling  upon  a  CONCAVE  surface,  if  they 
diverge  from  the  focus  of  parallel  rays,  become  parallel. 

(4.)  If  from  a  point  nearer  to  the  surface  than  that  focus, 
they  diverge  less  than  before  reflection. 

(5.)  If  from  a  point  between  that  focus  and  the  centre,  they 
converge,  after  reflection,  to  some  point  on  the  contrary  side  of 
the  centre,  and  further  from  the  centre  than  the  point  from 
which  they  diverged. 

(6.)  If  from  a  point  beyond  the  centre,  the  reflected  ra>& 
will  converge  to  a  point  on  the  contrary  side,  but  nearer  to  it 
than  the  point  from  which  they  diverged. 

(7.)  If  from  the  centre,  they  will  be  reflected  back  to  tht 
same  point  from  which  they  proceeded. 

How  are  objects         836.  As  a  necessary  consequence  of  the  laws 
teen  from  a  con-    which  have  now  been  recited,  it  may  be  stated, 
First,    in   regard  to  CONVEX  MIRRORS,  the   im- 
ages of  objects  invariably  appear  beyond  the  mirror ;  in  other 
.  they    are   virtual   images.     Secondly,  they  are  seen  in 
10 


NATURAL    PHILOSOPHY. 

their  natural  position,  and,  Thirdly,  they  are  smaller  than 
the  objects  themselves ;  the  further  the  object  is  from  the  mir- 
ror, and  the  less  the  radius  of  the  mirror,  the  smaller  the  image 
will  be.  If  the  object  be  very  remote,  its  image  will  be  in  the 
virtual  focus  of  the  mirror. 

837,  Secondly,  in  regard  to  CONCAVE  MIRKORS. 

(1.)  The  image  of  an  object  very  remote  from  a  concave  mir- 
ror, as  that  of  the  sun,  will  be  in  the  focus  of  the  mirror,  and 
the  image  will  be  extremely  small.*1 

(2.)  Every  object  which  is  at  a  distance  from  the  mirror 
greater  than  its  centre  produces  an  image  between  this  point 
and  the  focus  smaller  than  the  object  itself,  and  in  an  inverted 
position. 

(3.)  If  the  o'Mect  be  at  a  distance  from  the  mirror  equal  to 
the  length  of  its  radius,  then  the  image  will  be  at  an  equal  dis- 
tance from  the  mirror,  and  the  dimensions  of  the  image  will  be 
the  same  as  those  of  the  object,  but  its  position  will  be  inverted. 

(4.)  If  the  object  be  between  the  focus  and  the  centre  of 
curvature,  the  image  will  be  inverted,  and  its  size  will  much 
exceed  that  of  the  object. 

These  four  varieties  of  inverted  images,  produced  by  th* 
reflection  of  the  rays  of  light  from  concave  mirrors,  arc  some- 
times called  "physical  spectra." 

*  This  is  the  manner  in  which  concave  mirrors  become  burning-glasses. 
The  rays  of  the  sun  fail  upon  them  parallel  [see  No.  835],  and  they  are  all 
reflected  into  one  point,  called  the  focus,  where  the  light  and  heat  are  as 
much  greater  than  the  ordinary  light  and  heat  of  the  sun  as  the  area  of  the 
mirror  is  greater  than  the  area  of  the  focus.  It  is  related  of  Archimedes, 
that  he  employed  burning-mirrors,  two  hundred  years  before  the  Christian 
era,  to  destroy  the  besieging  navy  of  Marcellus,  the  Roman  consul.  His 
mirror  was,  probably,  constructed-  from  large  numbers  of  flat  pieces.  M. 
de  Vilette  constructed  a  burning-mirror  in  which  the  area  of  the  mirror  was 
seventeen  thousand  times  greater  than  the  area  of  the  focus.  The  heat  of  the 
sun  was  thus  increased  seventeen  thousand  times.  M.  Dufay  made  a  concave 
mirror  of  plaster  of  Paris,  gilt  and  burnished,  twenty  inches  in  diameter, 
with  which  he  set  fire  to  tinder  at  the  distance  of  fifty  feet.  But  the  most 
remarkable  thing  of  the  kind  on  record  is  the  compound  mirror  constructed! 
by  Butfon.  He  arranged  one  hundred  and  sixty-eight  small  plane  mirrors 
in  such  a  manner  as  to  reflect  radiant  light  and  heat  to  the  same  focus,  like 
one  large  concave  mirror.  With  this  apparatus  he  was  able  to  set  wood  on 
fire  at  the  distance  of  two  hundred  and  nine  feet,  to  melt  U>aJ  at  a  liun- 
dreu  feet,  and  silver  at  fifty  feet. 


OPTICS.  1329 

The  existence  and  position  of  these  spectra  may  easily  be  shown 
experimentally  thus  : 

Experiment. —  Hold  a  candle  opposite  to  a  concave  mirror,  at  ths 
distances  named  in  the  last  four  paragraphs  respectively.  The 
spectrum  can,  in  each  case,  be  received  on  a  white  screen,  which 
must  be  placed  at  the  prescribed  distance  from  the  mirror. 

Different  optical  instruments,  especially  reflecting  telesccpes, 
exhibit  the  application  of  these  spectra. 

(5.)  If  a  luminous  body,  as,  for  instance,  the  flame  of  an 
argand  lamp,  or  a  burning  coal,  be  placed  in  the  focus  of  a  con- 
cave mirror,  no  image  will  be  produced,  but  the  whole  surface 
of  the  mirror  will  be  illuminated,  because  it  reflects  in  parallel 
lines  all  the  rays  of  light  that  fall  upon  it.  This  may  be  made 
the  subject  of  an  experiment  so  simple  as  not  to  require  further 
explanation. ' 

'The  reflectois  of  compound  microscopes,  magic  lanterns  and  light- 
houses, by  means  of  which  the  light  given  by  the  luminous  bodj 
is  increased  and  transmitted  in  some  particular  direction  that  maj 
be  desired,  are  illustrations  of  the  practical  application  of  this  prin- 
ciple. 

(6.)  Lastly,  place  the  object  between  the  mirror  and  the 
focus,  and  the  image  of  the  object  will  appear  behind  the  mir- 
ror. It  will  not  be  inverted,  but  its  proportions  will  be  enlarged 
according  to  the  proximity  of  the  object  to  the  focus.  It  is 
this  circumstance  that  gives  to  concave  mirrors  their  magnifying 
powers,  and,  because  by  collecting  the  sun's  rays  into  a  focus 
they  produce  a  strong  heat,  they  are  called  burning-mirrors. 

838.  MEDIA,  OR  MEDIUMS,  AND  REFRAO 
What  is  a  Me-  TTON- —  A  Medium,*  in  Optics,  is  any  sub- 
dium  in  Optics?  stance,  solid  or  fluid,  through  which  light 

can  pass. 

What  is  refrac-        839.  When  light  passes  in  an  oblique 

direction  from  one  medium  into  another,  it 

is  turned  or  bent  from  its  course,  and  this  is  called  refrac- 

*  The  proper  plural  of  this  word  is  media,  although  mediums  is  frequently 
OM<1. 


230  NATURAL    PHILOSOPHY. 

twn.     The  property  which  causes  it  is   called 
bility 

840.  DIOPTRICS. —  That  part  of  the  sci- 
tri~.9?         l°P"    ence  of  Optics  which  treats  of  refracted  light 
k  called  Dioptrics. 

What  is  meant  ^^"  ^  medium,  in  Optics,  is  called  dense  or 
by  a  denser  and  rare  according  to  its  refractive  power,  and  not 
™rOt>ti  ™TlUm  according  to  its  specific  gravity.  Thus,  alcohol, 
and  many  of  the  essential  oils,  although  of  less 
specific  gravity  than  water,  have  a  greater  refracting  power, 
and  are,  therefore,  called  denser  media  than  water.  In  the  fol- 
lowing list,  the  various  substances  are  enumerated  in  the  order 
of  their  refractive  power,  or,  in  other  words,  in  the  order  of 
their  density  as  media,  the  last-mentioned  being  th«  densest, 
and  the  first  the  rarest,  namely :  air,  ether,  ice,  water,  alcohol, 
alum,  olive  oil,  oil  of  turpentine,  amber,  quartz,  glass,  molted 
sulphur,  diamond. 

842.  There  are  three  fundamental  laws  of 
What  are  the  T..  .  .  ,  .  ,  ,,  .,  ,  , 

fundamental       Dioptrics,   on  which  all  its  phenomena  de- 

Imos  of  Diop-    pend,  namely  : 

(1.)  When  light  passes  from  one  medium 
to  another  in  a  direction  perpendicular  to  the  surface,  it 
continues  on  in  a  straight  line,  without  altering  its  course. 

(2.)  When  light  passes  in  an  oblique  direction,  from  a 
rarer  to  a  denser  medium,  it  will  be  turned  from  its  course, 
and  proceed  through  the  denser  medium  less  obliquely,  and 
in  a  line  nearer  to  a  perpendicular  to  its  surface. 

(3.)  When  light  passes  from  a  denser  to  a  rarer  medium 
in  an  oblique  direction,  it  passes  through  the  rarer  medium 
in  a  more  oblique  direction,  and  in  a  line  further  from  a 
perpendicular  to  the  surface  of  the  denser  medium. 

843.  In  Fig.  130,  the  line  A  B  represents  a 
*£'      ray  of  light  passing  from  air  into  water,  in  a 
perpendicular  direction.     According  to  the  first 


OPTICS. 

[aw  stated  above,  it  will  continue  on  in  the  **•  1SO 

same  line  through  the  denser  medium  to  E. 


If  the  ray  were  to  pass  upward  through  the 
denser  medium,  the  water,  in  the  same  per- 
pendicular direction  to  the  air,  by  the  same 
law  it  would  also  continue  on  in  the  same 


straight  line  to  A. 

But,  if  the  ray  proceed  from  a  rarer  to  a  denser  medium,  in 
an  oblique  direction,  as  from  C  to  B,  when  it  enters  the  denser 
medium  it  will  not  continue  on  in  the  same  straight  line  to  D, 
but,  by  the  second  law,  stated  above,  it  will  be  refracted  or  bent 
out  of  its  course  and  proceed  in  a  less  oblique  direction  to  F 
which  is  nearer  the  perpendicular  ABE  than  D  is. 

Again,  if  the  ray  proceed  from  the  denser  medium,  the  water, 
to  the  rarer  medium,  the  air,  namely,  from  F  to  B,  instead  of 
pursuing  its  straight  course  to  G,  it  will  be  refracted  according 
to  the  third  law  above  stated,  and  proceed  in  a  more  oblique 
direction  to  C,  which  is  further  from  the  perpendicular  E  B  A 
than  G  is.  The  refraction  is  more  or  less  in  all 
tion  is  ^refrac  cases  m  proportion  as  the  rays  fall  more  or  less 
lion  in  all  cases  ?  obliquely  on  the  refracting  surface. 

844.  From  what  has  now  been  stated  with 
f  •  regard  to  refraction,  it  will  be  seen  that  many 
taking  the  depth  interesting  facts  may  be  explained.  Thus,  an 
if  water,  and  oar?  or  a  stick,  when  partly  immersed  in  water, 
appears  bent,  because  we  see  one  part  in  one 
medium,  and  tht  other  in  another  medium :  the  part  which  is  in 
the  water  appears  higher  than  it  really  is,  on  account  of  the 
refraction  of  the  denser  medium.  For  the  same  reason,  when 
we  look  obliquely  upon  a  body  of  water  it  appears  more  shallow 
than  it  really  is.  But,  when  we  look  perpendicularly  down- 
wards, we  are  liable  to  no  such  deception,  because  there  will  be 
no  refraction. 

845.  Let  a  piece  of  money  be  put  iito  a  cup  or  a  bowl,  and  the 
cup  and  the  eye  *  be  placed  in  such  a  position  that  the  side  of  the 
*»:}<  will  just  hfde  the  money  from  the  sight;  then,  keeping  the  ev 


232  NATURAL   PHILOSOPHY. 

directed  to  the  same  spot,  let  the  cup  be  filled  with  water, —  th» 
monoy  will  become  distinctly  visible. 

Why  do  we  not  846<  The  refraction  of  Hght  prevents  our 
see  the  sun,  moon  seeing  the  heavenly  bodies  in  their  real  situa- 
and  stars, intheir  ^jon 

The  light  which  they  send  to  us  is  refracted 
in  passing  through  the  atmosphere,  and  we  see  the  sun,  the 
stars,  &c.,  in  the  direction  of  the  refracted  ray.  In  conse- 
quence of  this  atmospheric  refraction,  the  sun  sheds  his  light 
upon  us  earlier  in  the  morning,  and  later  in  the  evening,  than 
we  should  otherwise  perceive  it.  And,  when  the  sun  is  actually 
below  the  horizon,  those  rays  which  would  otherwise  be  dissi- 
pated through  space  are  refracted  by  the  atmosphere  towards 
the  surface  of  the  earth,  causing  twilight.  The  greater  the 
density  of  the  air,  the  higher  is  its  refractive  power,  and,  conse- 
quently, the  longer  the  duration  of  twilight 

It  is  proper,  however,  here  to  mention  that  there  is  another  rea- 
son, why  we  do  not  see  the  heavenly  bodies  in  their  true  situ- 
ation. Light,  though  it  moves  with  great  velocity,  is  about  eight 
and  a  half  minutes  in  its  passage  from  the  sun  to  the  earth,  so  that 
\vhen  the  rays  reach  us  the  sun  has  quitted  the  spot  he  occupied 
on  their  departure  ;  yet  we  see  him  in  the  direction  of  those  rays, 
and,  consequently,  in  a  situation  which  he  abandoned  eight  minutes 
und  a  half  before".  The  refraction  of  light  does  not  affect  the  appear- 
ance of  the  heavenly  bodies  when  they  are  vertical,  that  is,  directly 
over  our  heads,  because  the  rays  then  pass  vertically,  a  direction 
incompatible  with  refraction. 

847.  When  a  ray  of  light   passes   from 
What    effect   is                    ,.  ,  if 

produced  when    one  medium  to  another,    and  through  that 

light  suffers  two  into  the  first  again,  if  the  two  refractions  be 
iijual  refrac-  ,  ,  .  •,.•»• 

f'ont  ?  J  equal,   and  in  opposite   directions,  no  sen- 

sible effect  will  be  produced. 

This  explains  the  reason  why  the  refractive  power  of  flat  window- 
glass  produces  no  effect  on  objects  seen  through  it.  The  rays  suffer 
two  refractions,  which,  being  in  contrary  directions,  produce  the 
game  effect  as  if  no  refraction  had  taken  place. 

848.  LENSES. — A  Lens  is  a  glass,  which, 
What  is  a  Lens?        .  ..  ,.       f  ' 

owing  to  its  peculiar  form,  causes  the  rays 


OPTICS. 


of  light  to  converge  to  a  focus,  or  disperses  them,  according 
to  the  laws  of  refraction. 

Explain  the  dif-        &49.  There  are  various  kinds  of  lenses, 
ferent  kinds  of    named  according  to  their  focus ;  but  they 
are  all  to  be  considered  as  portions  of  the 
internal  or  external  surface  of  a  sphere.     (See  par.  1480.) 

850.  A    single 
convex    lens    has 
one   side    flat  and 
the  other  convex ; 
as  A,  in  Fig.  131. 

851.  A   single 

concave  lens  is  flat  on  one  side  and  concave  on  the  other,  aa 
B  in  Fig.  131. 

852.  A  double  convex  lens  is  convex  on  both  sides,  as 
C,  Fig.  131. 

A  double  concave  le.ns  is  concave  on  both  sides,  as  D, 
Fig.  131. 

A  meniscus  is  convex  on  one  side  and  concave  on  the 
other,  as  E,  Fig.  131. 

What  is  the  853.  The  word  meniscus  is  derived  from  the 
°f  a  Greek  language,  and  means  literally  a  little 
moon.  This  term  is  applied  to  a  concavo-convex 
lens,  from  its  similarity  to  a  moon  in  its  early  appearance.  To 
this  kind  of  lens  the  term  periscopic  has  recently  been  applied, 
from  the  Greek  language,  meaning  literally  viewing  on  all  sides. 
When  the  concave  and  convex  sides  of  periscopic  glasses  are 
even,  or  parallel,  they  act  as  plane  glasses ;  but  when  the  sides 
are  unequal,  or  not  parallel,  they  will  act  as  concave  or  convex 
lenses,  according  as  the  concavity  or  the  convexity  is  the  greater, 

What  is  the  axis        854.  The  axis  of  a  lers  is  a  line  passing 
yf  a  lens?  through  the  centre  :  thu?  F  G,  Fig.  131,  is 

the  axis  of  all  the  five  lenses. 


Meniscus  ? 


NATURAL   PHILOSOPHY. 

85D.  The  peculiar  form   of  the   various 
lenses  ?  kinds  of  lenses  causes  the  light  which  passes 

through  them  to  be  refracted  from  its  course 
according  to  th)  laws  of  Dioptrics. 

It  will  be  remembered  that,  according  to  these  laws,  light,  in 
passing  from  a  rarer  to  a  denser  medium,  is  refracted  towards 
the  perpendicular  ;  and,  on  the  contrary,  that  in  passing  from  a 
denser  to  a  rarer  medium  it  is  refracted  further 
c.  ti°  TUfh  Wf  ^rom  ^e  PerPendicular.  I*1  order  to  estimate 
feet  of  a  lens  J  the  effect  of  a  lens,  we  must  consider  the  situa- 
tion of  the  perpendicular  with  respect  to  the 
surface  of  the  leas.  Now,  a  perpendicular,  to  any  convex  or 
concave  surface,  must  always,  when  prolonged,  pass  through 
the  centre  of  sphericity ;  that  is,  in  a  lens,  the  centre  of  the 
sphere  of  which  the  lens  is  a  portion.  By  an  attentive  observa- 
tion, therefore,  of  the  laws  above  stated,  and  of  the  situation  of 
the  perpendicular  on  each  side  of  the  lens,  it  will  be  found,  in 
general,  — 

(1.)   That  convex  lenses  collect  the  rays  into 
**>**,«*  magnify  objects  at  a  certain  dis- 
cave  lenses    re-    tance. 

*Pec  (2.)   That  concave  lenses  disperse   the  rays, 

and  diminish  objects  seen  through  them. 

What  is  the  fr-  856.  The  focal  distance  of  a  lens  is  the 
cal  distance  of  distance  from  the  middle  of  the  glass  to  the 
focus.  This,  in  a  single  convex  lens,  is  equal 
to  the  diameter  of  the  sphere  of  which  the  lens  is  a  portion, 
and  in  a  double  convex  lens  is  equal  to  the  radius  of  a 
sphere  of  which  the  lens  is  a  portion. 

857.  When  parallel  rays  *  fall  on  a  corx^ 
What  rays  will  ,  ,  ,        ,  .  ,     .  .,  .       .       .. 

pass  through  a    vex  ^ens?  those  only  which  fall  in  the  direc- 

lens  without  re-    tion  of  the  axis  of  the  lens  are  perpendicular 
to  its  surface,  and  those  only  will  continue 

*  The  rays  of  the  sun  are  considered  parallel  at  the  surface  of  the  earth. 
They  aie  not  so  in  reality,  but,  on  account  of  the  great  distance  of  that 
luminary,  their  divergency  is  SJ  small  that  it  is  altogether  inappreciable. 


OPTICS.  235 

on  in  a  straight  line  through  the  lens.  The  other  rays, 
falling  obliquely,  are  refracted  towards  the  axis,  and  will 
meet  in  a  focus.  (See  par.  1484) 

858.  It  is  this  property  of  a  convex  lens 
ci  ^/e W are  ^sun  w^c^  gives  it  its  power  as  a  burning-glass,  or 
glasses,  or  sun-glass.  All  the  parallel  rays  of  the  sun 

burning-glasses,  which  pass  through  the  glass  are  collected  to- 
conslructed  ?  ,,  .  ,,  „  ,  .  7  7 

gether  in  the  focus ;  and,  consequently,  the  heat 

at  the  focus  is  to  the  common  heat  of  the  sun  as  tJie  area  of  the 
glass  is  to  the,  area  of  the  focus.  Thus,  if  a  lens,  four  inches  in 
diameter,  collect  the  sun's  rays  into  a  focus  at  the  distance  of 
twelve  inches,  the  image  will  not  be  more  than  one-tenth  of  an 
inch  in  diameter;  the  surface  of  this  little  circle  is  1600  times 
less  than  the  surface  of  the  lens,  and  consequently  the  heat 
will  be  1600  timeb  greater  at  the  focus  than  at  the  lens. 

859.  The  following  effects  were  produced  by  a  large  lens,  or  burn- 
ing-glass, two  feet  in  diameter,  made  at  Leipsic  in  1691.  Pieces  of 
lead  and  tin  were  instantly  melted  ;  a  plate  of  iron  was  soon  ren- 
dered red-hot,  and  afterwards  fused,  or  melted ;  and  a  burnt  brick 
was  converted  into  yellow  glass.  A  double  convex  lens,  three  feet 
in  diameter,  and  weighing  two  hundred  and  twelve  pounds,  made  by 
Mr.  Parker,  in  England,  melted  the  most  refractory  substances'. 
Cornelian  was  fused  in  seventy-five  seconds,  a  crystal  pebble  in  six 
seconds,  and  a  piece  of  white  agate  in  thirty  seconds.  This  lens 
was  presented  by  the  King  of  England  to  the  Emperor  of  China. 

860.  If  a  convex  lens  have  its  sides  ground 
down  into  several  flat  surfaces,  it  will  present 
as  many  images  of  an  object  to  the  eye  as  it 
has  flat  surfaces.  It  is  then  called  a  Multiplying-glass.  Thus, 
if  cne  lighted  candle  be  viewed  through  a  lens  having  twelve 
flat  surfaces,  twelve  candles  will  be  seen  through  the  lens.  The 
principle  of  the  multiplying-glass  is  the  same  with  that  of  a 
crnvex  or  concave  lens. 

801.  The  following  effects  result  from  the  laws  of  refraction 
FACTS   WITH   REGARD   TO  CONVEX    SURFACES. — (1.)  Parallel  rays 
passing  out  of  a  rarer  into  a  denser  medium,  through  a  CONVEX  sur- 
face,  will  become  converging. 

'2.)    Divevginor  rays  will  be  made  to  diverge  less,  to  become  por- 
10* 


236  NATURAL    PHILOSOPHY. 

allel,  or  to  converge,  according  to  the  degree  of  divergency  before 
refraction,  or  the  convexity  of  the  surface. 

(3.)  Converging  rays  towards,  the  centre  of  convexity  will  sufibr 
no  refraction. 

(4.)  Rays  converging  to  a  point  beyond  the  centre  of  convexity 
will  be  made  more  converging. 

(5.)  Converging  rays  towards  a  point  nearer  the  surface  thac 
tne  centre  of  convexity  will  be  made  less  converging  by  refraction. 

[When  the  rays  proceed  out  of  a  denser  into  a  rarer  medium,  the 
reverse  occurs  in  each  case.] 

862.  FACTS  WITH  REGARD  TO  CONCAVE  SURFACES.  —  (1.)  Parallel 
rays  proceeding  out  of  a  rarer  into  a  denser  medium,  through  a 
CONCAVE  surface,  are  made  to  diverge. 

(2.)  Diverging  rays  are  made  to  diverge  more,  to  suffer  no 
refraction,  or  to  diverge  less,  according  as  they  proceed  from  a 
point  boyond  the  centre,  from  the  centre,  or  between  the  centre  and 
the 


(3.)  Don  verging  rays  are  made  less  converging,  parallel,  or  diverg- 
ing, accosting  to  their  .degree  of  convergency  before  refraction. 

803.  The  above  eight  principles  are  all  the  necessary  consequence 
of  the  operation  of  the  three  laws  mentioned  as  the  fundamental 
laws  of  Dioptrics.  The  reason  that  so  many  different  principles  are 
produced  by  the  operation  of  those  laws  is,  that  the  perpendiculars 
to  a  convex  or  concave  surface  are  constantly  varying,  so  that  no 
two  are  parallel.  But  in  flat  surfaces  the  perpendiculars  are  paral- 
lel ;  and  one  invariable  result  is  produced  by  the  rays  when  jpaes- 
ing  from  a  rarer  to  a  denser,  or  from  a  denser  to  a  rarer  medium, 
having  a  flat  surface. 

[When  the  rays  proceed  out  of  a  denser  into  a  rarer  medium,  the 
reverse  takes  place  in  each  case.] 

864.  Double  convex,  and  double  concave 

What  kinds  of       ,  ,  ,    .  , 

glasses  are  used  glasses,  or  lenses,  are  used  in  spectacles,  to 

in  spectacles,  remedy  the  defects  of  the  eye  :  the  former, 

and    for    what  -.                              •.                          a   ,          i 

purpose?  when  by  age  it  becomes  too  flat,  or  loses  a 

What  kinds  of    portion  of  its  roundness:  the  latter,   when 
erall  *  worn^b     V  anJ  otner  cause  it  assumes  too  i  ound  a 


old  persons?        form,  as  in  the  case  of  short-sighted  (or,  as 

"I  \  Tl  7  *      J       7,  O\? 

young  ?  m      y    tney   are    sometimes    called,    near-tighted) 
persons.     Convex  glasses  are  used  when  the 
eye  is  too  flat,  and  concave  glasses  when  it  is  too  round. 

These  lenses  or  glasses  are  generally  numbered,  by  opticians, 
according  to  their  degree  of  convexity  or.  concavity  ;  so  thai,  by 
knowing  the  number  that  fits  the  eye,  the  purchaser  can  generally 
bo  accommodated  without  the  trouble  af  trying  many  glajaes. 


uracs. 


"231 


806  THE  EYE.  —  The  eyes  of  all  animals  are  constructed  on  the 
same  principles,  with  such  modifications  as  are  necessary  to  adapt 
them  to  the  habits  of  the  animal.  The  knowledge,  therefore,  of  the 
construction  of  the  eye  of  an  animal  will  give  an  insight  of  the  con- 
struction of  the  eyes  of  all. 

~?  866.  The  eye  is  composed  of  a  number  of 

U/    what  is  \  ... 

tke  eye  com-    coats,  or  coverings,  within  which  are  enclosed 

posed?  a  jenSj  an(j  certain  humors,  in  the   shape  and 

performing  the  office  of  convex  lenses. 

What  are  the  different       86^.  The  different  parts  of  the  eye 

par  Is  of  the  eye  ?  are  : 

(1.)  The  Cornea. 

(2.)  The  Iris. 

(3.)  The  Pupil. 

(4.)  The  Aqueous  Humor. 


(6.)  The  Vitreous  Humor 
(7.)  The  Retina. 
(8.)  The  Choroid. 
(9.)  The  Sclerotica. 


(5.)  The  Crystalline  Lens.    |  (10.)  The  Optic  Nerve. 

Explain  868'   FiS'  132  rePresents  Fig-  132. 

Fig.  132.  a  front  view  of  the  eye,  in 
which  a  a  represents  the  Cornea,  or,  as 
it  is  commonly  called,  the  white  of  the 
eye ;  e  e  is  the  Iris,  having  a  circular 
opening  in  the  centre,  called  the  pupil, 
p,  which  contracts  in  a  strong  light,  and 
expands  in  a  faint  light,  and  thus  reg- 
ulates the  quantity  which  is  admitted 
to  the  tender  parts  in  the  interior 
of  the  eye. 

Explain  869'  *ig-  13S  reP" 

Fig.  133.     resents  a  side  view  of 

the  eye,  laid  open,  in  which  b  b 
represents  the  cornea,  e  e  the  iris, 
i  d  the  pupil, //the  aqueous  hu- 
aaor,  g  g  the  crystalline  lens,  'i  h 


NATbRAL    PHILOSOPHY. 

the  vitreous  humor,  i  i  i  i  i  the  retina,  c  c  the  choroid,  a  a  a 
a  a  the  scl erotica,  and  n  the  optic  nerve. 

Describe  the  $7 '0.  The  Cornea  forms  the  anterior  portion 
Cornea.  the  eye.  It  is  set  in  the  sclerotica  in  the  same 
•nanner  as  the  crystal  of  a  watch  is  set  in  the  case.  Its 
degree  of  convexity  varies  in  different  individuals,  and  in 
different  periods  of  life.  As  it  covers  the  pupil  and  the 
iris,  it  protects  them  from  injury.  Its  principal  office  is  to 
cause  the  light  which  reaches  the  eye  to  converge  to  the 
axis.  Part  of  the  light,  however,  is  reflected  by  its  finely  - 
polished  surface,  and  causes  the  brilliancy  of  the  eye. 

Describe  the  871.  The  Iris  is  so  named  from  its  being 

l™>  of  different  colors.     It  is  a  kind  of  circular 

curtain,  placed  in  the  front  of. the  eye,  to  regulate  the  quan- 
tity of  light  passing  to  the  back  part  of  the  eye.  It  has  a 
circular  opening  in  the  centre,  which  it  involuntarily  en- 
larges or  diminishes. 

872.  It  is  on  the  color   of  the   iris  that 

What  causes  a    the  color  of  the  eye  depends.     Thus  a  person 

person's  eyes  to    .         •  ».,"<  ^  t     ^      i  ^  ^        i 

be  black  blue  or  ls  sai(*  *°  nav^  black,   blue,   or  hazel   eyes 

gray,  <%c. .-'        according   as   the   iris   reflects   those   colors 

respectively. 

What  is  the  ^73.  The  Pupil  is  merely  the  opening  in  the 
Pupil?  iris,  through  which  the  light  passes  to  the  lens 

behind.  It  is  always  circular  in  the  human  eye,  but 
in  quadrupeds  it  is  of  different  shape.  W  nen  the  pupil  i& 
expanded  to  its  utmost  extent,  it  is  capable  of  admitting  ten 
times  the  quantity  of  light  that  it  does  when  most  con- 
tracted. 

874.  In  cats,  and  other  animals  which  are  paid 

Borne  'animals  to  see  *n  ^e  dark,  *he  power  of  dilatation  and  eon- 

M*  in  the        traction  is  much  greater;  it  is  computed  that  their 

pupils  may  receive  one  hundred   times  more  ligh< 


OPTICS.  239 

at  one  time  than  at  another.  That  light  only  which  passes  the 
pupil  can  be  of  use  in  vision  ;  that  which  falls  on  the  iris,  being 
reflected,  returns  through  the  cornea,  and  exhibits  the  color  of 
the  iris. 

When  we  come  from  a  dark  place  into  a  strong  light,  our  eyee 
suffer  pain,  because  the  pupil,  being  expanded,  admits  alarger  quan 
tity  of  light  to  rush  in,  before  it  has  had  time  to  contract.  And, 
when  we  go  from  a  strong  light  into  a  faint  one,  we  at  first  imagine 
ourselves  in  darkness,  because  the  pupil  is  then  contracted,  and  does 
not  instantly  expand. 

875.  The  Aqueous  Humor  is  a  fluid  as  clear 
Describe  the 

Aqueous  Hu-  as  the  purest  water.  In  shape  it  resembles  a 
meniscus,  and,  being  situated  between  the  cor- 
nea and  the  crystalline  lens,  it  assists  in  collecting  and 
transmitting  the  rays  of  light  from  external  objects  to  that 
lens. 

876.  The  Crystalline  Lens  is  a  transparent 
[Wiat  ts  the  J  r 
Crystalline     body,  in   the   form   of  a  double  convex  lens, 

Lens?  placed   between   the  aqueous  and  the  vitreous 

humors.  Its  office  is  not  only  to  collect  the  rays  to  a  focus 
on  the  retina,  but  also  to  increase  the  intensity  of  the  light 
which  is  directed  to  the  back  part  of  the  eye. 

T*-/,  ,  •  ,;  877.  The  Vitreous  Humor  (so  called  from  its 
\\hat  is  the  ^ 

Vitreous  Hu-  resemblance  to  melted   glass)    is   a   perfectly 
transparent  mass,  occupying  the  globe  of  tho 
eye.     Its  shape  is  like  a  meniscus,  the  convexity  of  which 
greatly  exceeds  the  concavity. 

878.  In  Fig.  134  the  shape  of  the 
aqueous  and  vitreous  humors  and  the  crys- 
tal] ine  lens  is  presented.  A  is  the  aqueous 
Humor,  which  is  a  meniscus,  B  the  crystal- 
line lens,  which  is  a  double  convex  lens, 
and  C  the  vitreous  humor,  which  is  aide  a 
meniscus,  whose  concavity  has  a  small  ir  radius  than  its  con- 
vexity. 


240  flATUKAL   PHILOSOPHY. 


What  is  tto  &7S.  The  Retina  is  the  seat  of  vision.  The 
Retina  ?  rajs  Of  light,  being  refracted  in  their  passage  by 
the  other  parts  of  the  eye,  are  brought  to  a  focus  in  the 
retina,  where  an  inverted  image  of  the  object  is  represented 
What  is  the  ^80.  The  Choroid  is  the  inner  coat  or  cover- 
Choroid?  jng  Of  the  eye.  Its  outer  and  inner  surface 
is  covered  with  a  substance  called  the  pigmentum  nigrum 
(or  black  paint).  Its  office  is,  apparently,  to  absorb  the 
rays  of  light  immediately  after  they  have  fallen  on  the  retina. 
It  is  the  opinion  of  some  philosophers  that  it  is  the  choroid, 
and  not  the  retina,  which  conveys  the  sensation  produced 
by  rays  of  light  to  the  brain. 

Describe  the  $81.  The  Sclerotica  is  the  outer  coat  of  the 
Sclerotica.  eye.  It  derives  its  name  from  its  hardness. 
Its  office  is  to  preserve  the  globular  figure  of  the  eye,  and 
defend  its  more  delicate  internal  structure.  To  the  sclero- 
tica  are  attached  the  muscles  which  move  the  eye.  It  re- 
ceives the  cornea,  which  is  inserted  in  it  somewhat  like  a 
watch-glass  in  its  case.  It  is  pierced  by  the  optic  nerve, 
which,  passing  through  it,  expands  over  the  inner  surface 
of  the  choroid,  and  thus  forms  the  retina. 

882.  The  Optic  Nerve  is   the   organ  which 

what  is  the  carr  jes  the  impressions  made  by  the  ravs  of 
Optic  Nerve?  J  * 

light  (whether  by  the  medium  of  the  retina,  or 

the  choroid)  to  the  brain,  and  thus  produces  the  sensation 
of  sight. 

What  optical  883.  The  eye  is  a  natural  camera  obscura 
instrument  r  »  ?•  o  n  r  i  11  •  f-n-i- 

foes  the  eye     isee  ™o.  805J,  and  the  -images  of  all  objects 

resemble?  seen  by  the  eye  are  represented  on  the  'retina 
in  the  same  manner  as  fne  forms  of  external  objects  are 
delineated  in  that  instrument. 

Explain        884.  Fig.  135  represents  only  those  parts  of  the  eye 
l**'      °'  which  are  most  essential   foi    the   explanation  of  the 


phenomenon  of  vision.  The  image  is  formed  thus :  The  :  ay? 
from  the  object  c  d,  diverging  towards  the  eye,  enter  the  cornea 
c,  and  cross  one  another  in  their  passage  through  the  crystalline 
lens  d,  by  which  they  are  made  to  converge  on  the  retina,  where 
they  form  the  inverted  image  /  e.  (See  par.  1488.) 

Hew  i.s  the  885.  ^ue  convexity  of  the  crystalline  humor  is 
convexity  of  increased  or  diminished  by  means  of  two  muscles, 

thecryttaltinefo     M  h  it  •    attached.     By   this  means,  the  focus 

lens  altered,  m  J 

and  for  what  of  the  rays  which  pass  through  it   constantly  falls 

purpose?  on  the  retina;  and  an  equally  distinct  image  is 
fcrmed,  both  of  distant  objects  and  those  which  are  near. 
How  can  you  886.  Although  the  image  is  inverted  on  the  re- 
"he^wrent  tma'  we  see  °^Jects  erec^  because  all  the  images 
position  of  formed  on  the  retina  have  the  same  relative  posi- 
oojccts  ?  tion  which  the  objects  themselves  have  ;  and,  as  the 
rays  all  cross  each  other,  the  eye  is  directed  upwards  to  receive 
the  rays  which  proceed  from  the  upper  part  of  an  object,  and 
downwards  to  receive  those  which  proceed  from  the  lower  part. 

887.  A  distinct  image  is  also  formed  on  the  re- 
Win!  do  we 

not  "see  double  tma  of  each  eye ;  but,  as  the  optic  nerves  of  the 
with  two  eyes  ?  two  eyes  unite,  or  cross  each  other,  before  they 
reach  the  brain,  the  impressions  received  by  the  two  nerves  are 
united,  so  that  only  one  idea  is  excited,  and  objects  a?e  seen 
single.  Although  an  object  nay  be  distinctly  seen  with  only 
one  eye,  it  has  been  calculated  that  the  use  of  both  eyes  makes 
u  difference  «>f  about  one-twelfth.  From  the  description  now 


NATURAL   PHILOSOPHY. 

given  of  the  eye,  it  may  be  seen  what  are  the  defects  wnich  art 
remedied  by  the  use  of  concave  and  convex  lenses,  and  how  the 
use  of  these  lenses  remedies  them. 

What  defects  888.  When  the  crystalline  humor  of  the  eye  is 

of  the  eye  are  too  rouna  tne  rays  of  light  which  enter  the  eye 

spectacles  de-  or                                            • 

signed  to  converge  to  a  focus  before  they  reach  the  retina, 

remedy  ?  and)  therefore,  the  image  will  not  be  distinct ;  and 

when  the  crystalline  humor  is  too  flat  (as  is  often  the  case  with 
old  persons),  the  rays  will  not  converge  on  the  retina,  but  tend 
to  a  point  beyond  it.  A  convex  glass,  by  assisting  the  converg- 
ency  of  the  crystalline  lens,  brings  the  rays  to  a  focus  on  the 
retina,  and  produces  distinct  vision.  • 

889.  The  eye  is  also  subject  to  imperfection  b\ 
For  what  de-  J  ,  •        A- 

fects  of  the     reason  0*    the  humors  losing  their  transparency 

eye  is  there  either  by  age  or  disease.  For  these  imperfection? 
no  remedy  ?  nQ  g]asses  Og-er  a  remedy,  without  the  aid  of  surgi- 
cal skill.  The  operation  of  couching  and  removing  cataracts 
from  the  eye  consists  in  making  a  puncture  or  incision  through 
which  the  diseased  part  may  escape.  Its  office  is  then  supplied 
by  a  lens.  If,  however,  the  operator,  by  accident  or  want  of 
skill,  permit  the  vitreous  humor  to  escape,  the  globe  of  the  eye 
immediately  diminishes  in  size,  and  total  blindness  is  the  inevi 
table  result 

What  is  a  ^®'  ^  gino^e  microscope  consists  simply  of 
singlemic.ro-  a  convex  lens,  commonly  called  a  magnifying- 
scope.  glass  ;  in  the  focus  of  which  the  object  is  placed 

and  through  which  it  is  viewed. 

891.  By  means  of  a  microscope  the  rays  of  light  from  an 
object  are  caused  to  diverge  less ;  so  that  when  they  enter  the 
pupil  of  the  eye  they  fall  parallel  on  the  crystalline  lens,  by 
which  they  are  refracted  to  a  focus  on  the  retina. 

Explain       892,    Fig.  136  represents  a  convex  lens,  or  single 

&•'       '  microscope,  C  P.     The  diverging  rays  from  tbe  object 

A  B  are  refracted  in  their  passage  through   the  leu*  C  P.  aii'J 


OPTICS. 


243 


made  to  fall  parallel  on 
the  crystalline  lens,  by 
which  they  are  refracted 
to  a  focus  on  the  retina 
R  R, ;  and  the  image  is 
thus  magnified,  because 
the  divergent  rays  are 
collected  by  the  lens  and 
carried  to  the  retina. 

893.   Those  lenses  or  microscopes  which  have 
What  glasses      ,,,.'«£  .  -  . 

have  the  great-    tne  shortest  focus  have  the  greatest  magnifying 

est  magnifying  power ;   and  those  which  are  the  most  bulging 
or  convex  have  the  shortest  focus.     Lenses  are 
made  small  because  a  reduction  in  size  is  necessary  to  an  increase 
of  curvature. 

What  is  a  double  894.  A  compound  microscope  consists 
microscope?  Of  £wo  convex  lenses,  by  one  of  which  a 
magnified  image  is  formed,  and  by  the  other  this  image  is 
carried  to  the  retina  of  the  eye. 

Explain  895.  Fig.  137  represents  the  effect  produced  by  the 
Fig.  137.  lenses  of  a  compound  microscope.  The  rays  which 
diverge  from  the  object  A  B  are  collected  by  the  lens  L  M  (called 
the  object-glass,  because  it  is  nearest  to  the  object),  and  form  an 

Fig.  1ST. 


invertcd  magnified  image  at  C  D.  The  rays  which  diverge  from 
this  image  are  collected  by  the  lens  N  O  (called  the  eye-glass, 
it  is  nearest  to  the  eye),  wWch  acts-  on  the  principle  of 


24A  NATURAL   PHILOSOPHY. 

the  single  microscope,  and  forms  still   another  image  on  the 
retina  RR. 


IW  /  '  fk  &$$•  The  solar  microscope  is  a  microscope 
solar  micro-  with  a  mirror  attached  to  it,  upon  a  movable 
wye  ;  joint,  which  can  be  so  adjusted  as  to  receive 

the  sun's  rays  and  reflect  them  upon  the  object.  It  con- 
sists of  a  tube,  a  mirror  or  •  looking-glass,  and  two  convex 
lenses.  The  sun's  rays  are  reflected  by  the  mirror  through 
the  tube  upon  the  object,  the  image  of  which  is  thrown  upon 
'  t  white  screen,  placed  at  a  distance  to  receive  it. 

897.  The  microscope,  as  above  described,  is  used  for  viewing 
transparent  objects  only.  When  opaque  objects  are  to  be  viewed, 
a  mirror  is  used  to  reflect  the  light  on  the  side  of  the  object  ; 
the  image  is  then  formed  by  light  reflected  from  the  object. 
instead  of  being  transmitted  through  it. 

898.  The  magnifying  power  of  a  single  mi- 
%?°£them*S~  croscope  is  ascertained  by  dividing  the  least 
of  singie  and  distance  at  which  an  object  can  be  distinctly 
double  micro-  geen  j^y  tne  nake(i  eyc  by  the  focal  distance  of 
scopes  ascer-  .  .  mi  .  .  , 

tained?  ™ie  ^Gns'   This,  in  common  eyes,  is  about  seven 

inches.  Thus,  if  the  focal  distance  of  a  lens 
be  only  £  of  an  inch,  then  the  diameter  of  an  object  will  be 
magnified  28  times  (because  7  divided  by  £  is  the  same  as  7 
multiplied  by  4),  and  the  surface  will  be  magnified  784  times. 

The  magnifying  power  of  the  compound  microscope  is  found 
in  a  similar  manner,  by  ascertaining  the  magnifying  power,  first 
of  one  lens,  and  then  of  the  other. 

The  magnifying  power  of  the  solar  microscope  is  in  propor- 
tion as  the  distance  of  the  image  from  the  object-glass  is 
greater  than  that  of  the  object  itself  from  it.  Thus,  if  the  dis- 
tance of  the  object  from  the  object-glass  be  £  of  an  inch,  and 
the  distance  of  the  image,  or  picture,  on  the  screen,  be  *en  feet. 
.ir  120  inches,  the  object  will  be  magnified  in  length  4bO  times 
or  in  surface  280,000  times, 


OPTICS. 


245 


A  lens  may  be  caused  to  magnify  or  to  aiminish  an  object.  If  the 
jbject  be  placed  at  a  distance  from  the  focus  of  a  lens,  and  the  im- 
age be  formed  in  or  near  the  focus,  the  image  will  be  diminished  ; 
but,  if  the  object  be  placed  near  the  focus,  the  image  will  be  mag- 
nified. 

What  is  the  Mag-  The  Magic  Lantern  is  an  instrument  con- 
k  Lantern?  structed  on  the  principle  of  the  solar  micro- 
scope, but  the  light  is  supplied  by  a  lamp  instead  of  the 
sun. 

899.  The  objects  to  be  viewed  by  the  magic  lantern  are  gener- 
ally painted  with  transparent  colors,  on  glass  slides,  which  are 

Fig.  138. 


received  into  an  opening  in  the  front  of  the  lantern.  The  light 
from  the  lamp  in  the  lantern  passes  through  them,  and  carries 
the  pictures  painted  on  the  slides  through  the  lenses,  by  means 
of  which  a  magnified  image  is  thrown  upon  the  wall,  on  a  white 
surface  prepared  to  receive  it. 

Fig.  138  represents  the  magic  lantern.  The 
rays  of  light  from  the  lamp  are  received  upon 
the  concave  mirror  e,  and  reflected  to  the  con- 
vex lens  c,  which  is  called  the  condensing  lens,  because  it  con- 
centrates a  large  quantity  of  light  upon  the  object  painted  on 
the  slide,  inserted  at  b.  The  rays  from  the  illuminated  object 
at  b  are  carried  divergent  through  the  lens  a,  forming  an  imagi 
on  the  screen  at/.  The  image  will  increase  or  diminish  in  sizo 
in  proportion  to  the  distance  of  the  screen  from  the  lens  a. 


Describe   Fig. 
138. 


1Mb  NATURAL    PHILOSOPHY. 

900.  DISSOLVING    VIEWS.  —  The    exhibition 

Hair  are"  Dis-    cai}ed  «  Dissolving  Views  "  is  made  by  means 

srlving  Views         „  ,  .    ,  „          , 

t "presented?          °*  *wo  rnagic  lanterns  of  equal  power,  so  as  to 

throw  pictures  of  the  same  magnitude  in  the 
Kime  position  on  the  screen.  By  the  proper  adjustment  of 
sliding  tubes  and  shutters,  one  picture  on  the  screen  is  made 
brighter  while  the  other  becomes  fainter,  so  that  the  one  seems 
to  dissolve  into  the  other.  In  the  hands  of  a  skilful  artist  # 
this  is  an  exhibition  of  the  most  pleasing  kind. 

901.  TELESCOPES.  —  A -Telescope  is   an 

What  is  a  Tel-     .  t    ,.         .      .         ,.  x  ,  / 

tscope?  instrument  tor  viewing  distant  objects,  and 

causing  them  to  appear  nearer  to  the  eye. 

How  are  tele-  ^2.  Telescopes  are  constructed  by  placing 
scopes  construct-  lenses  of  different  kinds  within  tubes  that  slide 
*"'  within  each  other,  thus  affording  opportunity 

of  adjusting  the  distances  between  the  lenses  within. 

903.  They  are  also  constructed  with  mirrors,  in  addition  to 
the  lenses,  so  that,  instead  of  looking  directly  at  an  object,  the 
eye  is  directed  to  a  magnified  image  of  the  object,  reflected 
from  a  concave  mirror.  This  has  given  rise  to 
How  many  kinds  ^ne  two  distinctions  in  the  kinds  of  telescopes 
there?  m  common  USG>  called  respectively  the  Refract- 

ing and  the  Reflecting  Telescope. 

How  is  the  Re-       904.  The   Refracting    Telescope  is  con- 
scopT^onstruci-  structed  with  lenses  alone,  and  the  eye  is 
d?  *     directed  toward  the  object  itself. 

905.  The   Reflecting   Telescope   is   con- 

How  does  a  Re-  -.      .  •,  .  -,  •>• 

fleeting    Tele-     structed  with  one.  or  more  mirrors,  in  addi- 

*  Mr.  John  A.  Whipple,  of  this  city,  has  given  several  exhibition!*  of 
this  kind,  with  great  success.  A  summer  scene  seemed  to  dissolve  into  tlie 
same  scene  in  mid-winter  ;  a  daylight  view  was  gradually  made  to  faint 
successively  into  twilight  and  moonshine;  and  many  changes  of  a  most  in- 
teresting nature  showed  how  pleasing  an  exhibition  might  be  made  by  o 
skiiful  combination  of  science  and  art 


OPTICS. 


scope  differ j  rorr.    tion  tc   the  lenses;    and  the  image  oi   the 
object,  reflected  from  a   >concave  mirror,  is 
seen,  instead  of  the  object  itself. 

906.  Each  of  these  kinds  of  telescope  has  its  respective  advan- 
tages, but  refracting  telescopes  have  been  so  much  improved  that 
they  have  in  some  degree  superseded  the  reflecting  telescopes. 

What    is    an  ^07.  Among  tne  improvements  which  have 

Achromatic  Tele-  been  made  in  the  telescope,  may  be  mentioned, 
sc°Pe-  as  the  most  important,  that  peculiar  construc- 

tion of  the  lenses  by  which  they  are  made  to  give  a  pencil  of 
white  light,  entirely  colorless.  Lenses  are  generally  faulty  in 
causing  the  object  to  be  partly  tinged  with  some  color,  which  is 
imperfectly  refracted.  The  fault  has  been  corrected  by  employ- 
ing a  double  object-glass,  composed  of  two  lenses  of  different 
refracting  power,  which  will  naturally  correct  each  other.  The 
telescopes  in  which  these  are  used  are  called  Achromatic.  Com- 
mon telescopes  have  a  defect  arising  from  the  convexity  of  the 
object-glass,  which,  as  it  is  increased,  has  a  tendency  to  tinge 
the  edges  of  the  images.  To  remedy  this  defect,  achromatic 
lenses  were  formed  by  the  union  of  a  convex  lens  of  crown 
glass  with  a  concave  lens  of  flint  glass.  Owing  to  the  difference 
of  the  refracting  power  of  these  two  kinds  of  glass,  the  images 
became  free  from  color  and  more  distinct;  and  hence  the  glasses 
which  produce  them  were  called  Achromatic,  that  is,  free  from 
color.  (See  pars.  1509-1511.) 

Lenses  are  also  subject  to  another  imperfection,  called  spheri- 
cal aberration,  arising  from  the  different  degrees  of  thickness 
in  the  "/nitre  and  edges,  which  causes  the  rays  that  are  refracted 
through  them  respectively,  to  come  to  different  focuses,  on  ac- 
count of  the  greater  or  less  refracting  power  of  these  parts,  con- 
sequent on  their  difference  in  thickness.  To  correct  this  defect, 
tenses  have  been  constructed  of  gems  and  crystals,  &c.,  which 
have  a  higher  refractive  power  than  glass,  and  require  less 
sphericity  to  produce  equal  effects. 

What  is  the  sim-  908-  T^e  simplest  form  of  the  telescope  coii- 
pkst  form  of  the  sists  of  two  convex  lenses,  so  combined  as  to 
ttkscove?  increase  the  angle  of  vision  under  which  th« 


248 


NATURAL    PIIILOSC  PIIY. 


Object-glass,  and 
which  the  Eye- 
glass, of  a  iele- 


object  is  seen.     The  lenses  are  so  placed   that  the  distance 
between  them  may  be  equal  to  the  sum  of  their  focal  distances 

Which  is  the  f\n^ 

909.  The  lens  nearest  to  the  eye  is  culled 

the  Eye-glass,  and  that  at  the  other  extrem- 
ity is  called  the  Object-glass. 

910.  Objects  seen  through  telescopes  of  tlip 
construction  (namely,   with   two  glasses    only) 
are  always  inverted,  and  for  this  .reason  this 
kind   of  instrument  is  principally  used  for  as- 
tronomical purposes,  in  which  the  inversion  of 

the  object  is  immaterial. 

What  is  the  dif-        91L  ThQ   common   day   telescope,   or    spy- 
ference   between    glass,  is  an  instrument  of  the  same  sort,  with 

a   day    and    a    foe    addition    of    two,  or    even    three    or  four 
night  telescope? 

glasses,  lor  the  purpose  ol  presenting  the  object 

upright,  increasing  the  field  of  vision,  and  diminishing  the  aber- 
ration caused  by  the  dissipation  of  the  rays. 

912.    Fig.  139    represents    the    parts    of   an 
Explain  Fig.      astrOnomical   telescope.      It  consists  of  a  tube 

lot/* 

A  B  C  D,  containing  two  glasses,  or  lenses. 
The  lens  A  B,  having  a  longer  focus,  forms  the  object-glass ; 
the  other  lens  D  C  is  the  eye-glass.  The  rays  from  a  very 


Pig.  139. 


scope , 


How  are  objects 
teen  throu  ^h  tel- 
escopes   of    the 
simplest   con- 
struction * 


distant  body,  as  a  star,  and  which  may  be  considered  parallel  to 
each  other,  are  refracted  by  the  object-glass  A  B  to  a  focus  at 
K.  The  image  is  then  seen  through  the  eye-glass  D  C,  magni- 
fied as  many  times  as  the  focal  length  of  the  eye-glass  is  con- 
tained in  the  focal  length  of  the  object-glass.  Thus,  if  the  focal 
length  of  the  eye-glass  D  C  be  contained  100  times  in  that  of 


OPTICS. 


the  object-glass  A  B,  the  star  will  be  seen  magniLed  100  times. 
It  will  be  seen,  by  the  figure,  that  the  image  is  inverted ;  for 
the  ray  M  A,  after  refraction,  will  bs  seen  in  the  direction  0  O, 
and  the  ray  N  B  in  the  direction  D  P.  (See  par.  1508.) 

913.  Fig.  140  represents  a  day-glass,  or  ter- 
restrial telescope,  commonly  called  a  spy-glass. 
This,  likewise,  consists  of  a  tube  A  B  H  G, 
containing  four  lenses,  or  glasses,  namely,  A  B,  C  D,  E  F,  and 
G  H.  The  lens  A  B  is  the  object-glass,  and  G  H  the  eye-glass. 
The  two  additional  eye-glasses,  E  F  and  C  D,  are  of  the  same 
size  and  shape,  and  placed  at  equal  distances  from  each  other, 

fig.  140. 


Explain    Fig. 
140. 


0    H 


in  such  a  manner  that  the  focus  of  the  one  meets  that  of  the 
next  lens.  These  two  eye-glasses  E  F  and  C  D  are  introduced 
for  the  purpose  of  collecting  the  rays  proceeding  from  the  in- 
verted image  M  N,  into  a  new  upright  image,  between  G  H  and 
E  F ;  and  the  image  is  then  seen  through  the  last  eye-glass  G  H, 
under  the  angle  of  vision  P  0  Q.  (See  par.  1511.) 

Opera  Glasses  are  constructed  on  the  prin- 
era  Classes  *  ^~  c^e  °^  ^e  refracting  telescope.  They  are  in 
fact,  nothing  more  than  two  small  telescopes, 
united  in  such  a  manner  that  the  eye-glasses  of  each  may  be 
moved  together,  so  as  to  be  adjusted  to  the  eyes  of  different 
persons.  (See  par.  1512.) 

Of  what  does  the  ^14.  THE  REFLECTING  TELESCOPE. — The  Re- 
fafating  Tel-  fleeting  Telescope,  in  its  simplest  form,  con- 
escope  consist?  gisted  of  ft  concave  mirror  and  a  convex 

eye-glass.     The  mirror  throws  an  image  of  the  object,  and  the 
s  views   that   image  under   a   larger  angle  of  vision. 


•J50 


NATURAL    PHILOSOPHY. 


This  instrument  was  subsequently  improved  by  Newton,  and 
since  him  by  Cassegrain,  Gregory,  Hadley,  Short,  and  th<» 
Herschels. 

915.  Fig.  141  represents  the  Gregorian 
l&  Telescope.  It  consists  of  a  large  tube,  con 
taining  two  concave  metallic  mirrors,  and  two 
plano-convex  eye-glass 3S.  The  rays  from  a  distant  object  are 
received  through  the  open  end  of  the  tube,  and  proceed  from  r  t 

Fig.  141. 


Explain 
141. 


>       B 


to  r  r,  at  the  large  mirror  A  B,  which  reflects  them  to  a  focus 
at  <7,  whence  they  diverge  to  the  small  mirror  C,  which  re- 
flects them  parallel  to  the  eye-glass  F,  through  a  circular  aper- 
•ure  in  the  middle  of  the  mirror  A  B.  The  eye-glass  F  col- 
lects those  reflected  rays  into  a  new  image  at  I,  and  this  image 
Is  seen  magnified  through  the  second  eye-glass  G. 

It  is  thus  seen  that  the  mirrors  bring  the  object  near  to  the 
eye,  and  the  eye-glasses  magnify  it.  Reflecting  telescopes  are 
attended  with  the  advantage  that  they  have  greater  magnifying 
power,  and  do  not  so  readily  decompose  the  light.  It  has 
already  been  stated  that  the  improvements  in  refractors  have 
given  them  the  greater  advantage.  (See  par.  1514.) 

How  does  the        916.  The  Cassegrainian  telescope  differs  from 

iaTteScone  that  which  haS  been  described'  in  having  tkp 
differ  from  smaller  mirror  convex.  This  construction  is  at- 
the  Gregorian  ?  tended  with  two  advantages ;  first,  it  is  superior 
in  distinctness  of  its  images,  and,  second,  it  dispenses  with  the 
necessity  of  so  long  a  tube. 


OPTICS.  251 

917.  The  talescopes  of  Herschel  and    of  Lord 
WJiat  peat- 
liaritus  are    -Kosse  dispense  with  the  smaller  mirror.     This  is 

there  in  the  done  by  a  slight  inclination  of  the  large  mirror,  so 
Herschel  and  as  *°  tnrow  tne  image  on  one  side,  where  it  is  viewed 
the  Earl  of  by  the  eye-glass  The  observer  sits  with  his  back 
Rosse?  towards  the  object  to  be  viewed.  Herschel's  gigan- 
tic telescope  was  erected  at  Slough,  near  Windsor,  in  1789.  The 
diameter  of  the  speculum  or  mirror  was  four  feet,  and  the  mir- 
ror weighed  2118  pounds ;  its  focal  distance  was  forty  feet.  (See 
par.  1514.) 

918.  The  telescope  of  Lord  Rosse  is  the  largest  that  has  ever 
been  constructed.  The  diameter  of  the  speculum  is  six  feet,  aud 
its  focal  distance  fifty-six  feet.  The  diameter  of  the  tube  is  seven 
feet,  and  the  tube  and  speculum  weigh  more  than  fourteen  tons. 
The  cost  of  the  instrument  was  about  $60,000. 

The  telescope  now  belonging  to  Harvard  University  is  a  refractor. 
It  is  considered  one  of  the  best  instruments  ever  constructed. 

What  is  919.   CHROMATICS.— That  part  of  the  sci- 

Chromatics?  ence  Of  Optics  which  relates  to  colors  is 
called  Chromatics. 

Of  what  is  light  920.  Light  is  not  a  simple  thing  in  its 
composed?  nature,  but  is  composed  of  rays  of  different 
colors,  each  of  which  has  different  degrees  of  refrangibility, 
and  has  also  certain  peculiarities  with  regard  to  reflection. 

Of  what  color  ^^'  ^ome  substances  reflect  some  of  the 
are  bodies  rays  that  fall  upon  them  and  absorb  the  others, 
composed?  gome  appear  to  reflect  all  of  them  and  absorb 
none,  while  others  again  absorb  all  and  reflect  none.  Hence, 
bodies  in  general  have  no  color  of  themselves,  independent 
of  light,  but  every  substance  appears  of  tint  color  which  it- 
reflects. 

What  are  ^^*  White  ^s  a  ^ue  mixture  of  all  colors  in 

white  and       nice  and  exact  proportion.     When  a  body  re- 
flects all  the  rays  that  fall  upon  it,  it  will  ap- 
pear white,  and  the  purity  of  the  whiteness  depends  on  the 
perfcc  tiiess  of  the  reflection. 
11 


NATURAL    PHILOSOPHY. 

923.  Black  is  the  deprivation  of  all  col(  r,  and, 

body  reflects  none  of  the  rays  that  fall  upan  it,   it  will 
appear  black. 

924.  Some  bodies  reflect  two  or  more  colors  either  partially 
or  perfectly,  and  they  therefore  present  the  varied  hues  which 
we   perceive,  formed   from    the   mixture   of  rays   of  different 
colors.^ 

What  are  the  925.  The  colors  which  enter  into  the  composi- 
2^??i  °f  tion  of  light,  and  which  possess  diiferent  degrees 
of  refrangibility,  are  seven  in  number,  namely, 
red,  orange,  yellow,  green,  blue,  indigo,  and  violet. 
What  is  a  926.  A  Prism  is  a  solid,  triangular  piece  of 
Prism?  highly- polished  glass. 

927.  A  prism  which  will  answer  the  same  purpose  as  a  solid  one 
may  be  made  of  three  pieces  of  plate  glass,  about  six  or  eight  inches 
long  and  two  or  three  broad,  joined  together  at  their  edges,  and 
made  water-tight  by  putty.  The  ends  may  be  fitted  to  a  triangular, 
piece  of  wood,  in  one  of  which  an  aperture  is  made  by  which  to  till 

*  When  the  eye  has  become  fatigued  by  gazing  intently  on  any  object, 
of  a  red  or  of  any  other  color,  the  retina  loses,  to  some  extent,  its  sensitive- 
ness to  that  color,  somewhat  in  the  same  manner  that  the  ear  is  deafened  for 
a  moment  by  an  overpowering  sound.  If  that  object  be  removed  and 
another  be  presented  to  the  eye,  of  a  different  color,  into  the  composition  of 
which  red  enters,  the  eye,  insensible  to  the  red,  will  perceive  the  other 
colors,  or  the  compound  color  which  they  would  form  by  the  omission  of  the 
red,  and  the  object  thus  presented  would  appear  of  that  color.  The  truth 
of  this  remark  may  be  easily  tested.  Fix  the  eye  intently  for  some  time  on 
a  red  wafer  on  a  sheet  of  white  paper.  On  removing  the  wafer,  the  white 
disk  beneath  it  will  transmit  all  the  colors  of  white  ligh  but  the  eye, 
insensible  to  the  red,  will  perceive  the  blue  or  green  colors  at  the  other  end 
of  the  spectrum,  and  the  other  spot  where  the  red  wafer  was  will  appear 
of  a  bluish-green,  until  the  retina  recovers  its  sensibility  for  red  light.  Th« 
colors  thus  substituted  by  the  fatigued  eye  are  called  the  accidental  color. 

The  accidental  colors  of  the  seven  prismatic  colo -8,  together  with  blacfc 
and  white,  are  as  follows  : 

Accident il  Coltr 

Red .  Bluish  Green. 

Orange Blue. 

Yellow Indigo. 

Green Violet  reddisn. 

Indigo Orange  red. 

Violet Orange  yellow. 

Black White. 

White  .  .  Bkick 


OPTICS. 


25?. 


it  with  water,  and  thus  to  give  it  the  appearance  and  the  refractive 
power  of  a  solid  prism. 


928.  When  light  is  made  to  pass  through  a 
prism,  the  different-colored  rays  are  refracted 
or  separated,  and  form  an  image  on  a  screen  or 
wall,  in  which  the  colors  will  be  arranged  in 
the  order  just  mentioned. 

929.  Fig.  142  represents  rays  of  light  passing  from 
•f*m     '  the  aperture,  in  a  window-shutter  A   B,  through  the 
prism  P.     Instead  of  continuing  in  a  straight  course  to  E,  and 
there  forming  an  image,  they  will  be  refracted,  in  their  passage 
through  the  prism,  and  form  an  image  on  the  screen  G  D.     But, 

Fig.  142.  A 


What  effect 
has  a  prism 
on  the  light 
that  passes 
through  it  ? 


Explain 


as  the  different-colored  rays  have  different  degrees  of  refrangi- 
bility,  those  which  are  refracted  the  least  will  fall  upon  the 
lowest  part  of  the  screen,  and  those  which  are  refracted  the  most 
will  fall  upon  the  highest  part.  The  red  rays,  therefore,  suffer- 
ing the  smallest  degree  of  refraction,  fall  on  the  lowest  part  of 
the  screen,  and  the  remaining  colors  are  arranged  in  the  order 
of  their  refraction.  (See  par.  1491.) 

930.  It  is  supposed  that  the  red  rays  are  refracted  the  least,  on 
account  of  their  greater  momentum  ;  and  that  the  blue,  indigo  and 
violet*  are  refracted  the  most,  because  they  have  the  least  momentum. 
The  same  reason,  it  is  supposed,  will  account  for  the  red  appoar- 
ance  of  the  sun  through  a  fog,  or  at  rising  and  setting.  Ihe  in- 
creased quantity  of  the  atmosphere  which  the  oblique  rays  must 
traverse,  and  its  being  loaded  with  mists  and  vapors,  which  are 
usually  formed  at  those  times,  prevents  the  other  rays  from  reach- 
ing us. 

A  similar  reason  will  account  fir  the  blue  appearance  of  the  ?!»;/. 


254  NATURAL    PHILOSOPHY. 

As  these  rays  hive  less  momentum,  they  cannot  traverse  the  atino*- 

Ehere  so  readily  as  the  other  rays, .and  they  are,  therefore,  reflected 
ack  to  our  eyes  by  the    atmosphere.     If  the  atmosphere  did  not 
reflect  any  rays,  the  skies  would  appear  perfectly  black, 

931.  If  the  colored  rays  which  have  been  sepa- 
How  can  the  .  r 

rar*  refract-    rated   by  a  pnsi:    fall    upon  a  convex   lens,  they 

ed  by  a  prism  will  converge  to  a  focus,  and  appear  white.     Hence 
it  appears  that  white  is  not  a  simple  color,  but  if- 
produced  by  the  union  01  several  colors. 

932.  The  spectrum  formed  by  a  glass  prism   being  divided 
in+o  360  parts,  it  is  found  that  the  red  occupies  45  of  those  parts, 
the  orange  27,  the  yellow  48,  the  green  60,  the  blue  60,  the 
indigo  40,  and  the  violet  80.     By  mixing  the  seven  primitive 
colors  in  these  proportions,  a  white  is  obtained  ;  but,  on  account 
of  the  impurity  of  all  colors,  it  will  be  of  a  dingy  hue.     If  the 
colors  were  more  clearly  and  accurately  defined,  the  white  thus 
obtained  would  appear  more  pure  also.     An  experiment  to  prove 
what  has  just  been  said  may  be  thus  performed  :    Take  a  circular 
piece  of  board,  or  card  and  divide  it  into  parts  by  lines  drawn 
from  the  centre  to  the  circumference.     Then,  having  painted  the 
seven  colors  ir,  the  proportions  above  named,  cause  the  board  to 
revolve  rapidly  around  a  pin  or  wire  at  the  centre.     The  board 
will  then   appear  of  a  white  color.     From  this  it  is  inferred 
that  the  whiteness  of  the  sun's  light  arises  from  a  due  mixtiu-e 
of  all  the  primary  colors.      (See  par.  1492.) 

933.  The  colors  of  all  bodies  are  either  the  simple  colors,  as 
refracted  by  the  prism,  or  such  compound  colors  as  arise  from  a 
mixture  of  two  or  more  of  them.     (See  par.  1498.) 

934.  From  the  experiment  of    Dr.  Wollaston, 
What  are  the  . 
three  simple    xt  appears  that  the  seven  colors  formed  by  the  prism 

colors?  may  be  reduced  to  four,  namely,  red,  green,  blue, 

and  violet ;  and  that  the  other  colors  are  produced  by  combina- 
tions of  these,  but  violet  is  merely  a  mixture  of  blue  and  red, 
and  green  is  a  mixture  of  blue  and  yellow.     A  better  division 
of  the  simple  colors  is  blue,  yellow,  and  red.     (See  par.  1502.) 
935.  Light  is  found  to  possess  both  heat  and  chemical  udioa. 


OPTICS.  255 

The  prismatic  spectaim  presents  some  remaikable  phenomena  with 
regard  to  these  qualities  :  for,  while  the  red  rays  appear  to  be  tna 
seat  of  the  maximum  of  heat,  the  violet,  on  the  contrary,  are  the 
apparent  se,\t  of  the  maximum  of  chemical  action. 

036.  Light,  from  whatever   source   it   proceeds,  is   of  the   same 
nature,  composed  of  the  various-colored  rays ;  and   although  some 
substances  appear  differently  by  candle-light  from  what  they  appear 
by  day,  this  rtault  may  be  supposed  to  arise  from  the  weakness  or 
want  of  purity  in  artificial  light. 

037.  There  can  be  no  light  without  colors,  and  there  can  be  no  colors 
without  light. 

938.  That  the  above  remarks  in  relation  to  the  colors  of  bodies 
are  true,  may  be  proved  by  the  following  simple  experiment.     Place 
a  colored  body  in  a  dark  room,  in  a  ray  of  light   that  has  been  re- 
fracted by  a  prism  ;  the  body,  of  whatever  color  it  naturally  is,  will 
appear  of  the  color  of  the  ray  in  which   it  is  placed ;  for,  since  it 
receives  no  other  colored  rays,  it  can  reflect  no  others. 

939.  Although  bodies,  from  the  arrangement  of  their  particles, 
have  a  tendency  to  absorb  some  rays  and  reflect  others,  they  are 
not  so  uniform  in  their  arrangement  as  to  reflect  only  pure  rays  of 
one  color,  and  perfectly  absorb  all  others  ;  it  is  found,  on  the  con- 
trary, that  a  body  reflects  in  great  abundance  the  rays  which  deter- 
mine its  color,  and  the  others  in  a  greater  or  less  degree  in  propor- 
tion as  they  are  nearer  or  further   from  its  color,  in  the  order  of 
refrangibility.     Thus,  the  green  leaves  of  a  rose  will  reflect  a  few  of 
the  red  rays,  which  will  give  them  a  brown  tinge.     Deepness  of 
color  proceeds  from  a  deficiency  rather  than  an  abundance  of  reflect- 
ed rays.     Thus,  if  a  body  reflect  only  a  few  of  the  green  rays,  it 
will  appear   of  a  dark  green.     The  brightness  and  intensity  of  a 
color  shows  that  8  great  quantity  of  rays  are  reflected.     That  bodies 
sometimes  change  -their  color,  is  owing  to  some  chemical    change 
which    takes  pla^e   in   the   internal   arrangement   of  their   parts, 
whereby   they   looe  their   tendency   to   reflect   certain   colors,  and 
acquire  the  power  of  reflecting  others. 

How  is  a  rain-  940.  The  rainbow  is  produced  by  the  re- 
bow  produced  ?  fraction  of  the  sun's  rays  in  their  passage 
through  a  shower  of  rain ;  each  drop  of  which  acts  as  a 
prism  in  separating  the  colored  rays  as  they  pass  through  it. 

941.  This  is  proved  by  the  following  considerations:  First, 
a  rainbow  is  pcver  seen  except  when  rain  is  falling  and  the  sun 
shining  at  the  same  time  ;  and  that  the  sun  and  the  bow  are 
always  in  opposite  parts  of  the- heavens  ;  and,  secondly,  that  the 
game  appearance  may  be  produced  artificially,  by  means  of  water 
thrown  into  the  air,  when  the  spectator  fc  placed  in  a  proper 


ICATUllAL    PHILOSOPHY. 

position,  with  his  back  to  the  sun ;  and,  thirdly,  that  a  simitar 
bow  is  generally  produced  by  the  spray  which  arises  from  large 
cataracts  or  waterfalls.  The  Falls  of  Niagara  afford  a  beautiful 
exemplification  of  the  truth  of  this  observation.  A  bow  is 
always  seen  there  when  the  sun  is  clear  and  the  spectator's  back 
is  towards  the  sun.  (See  par.  1501.) 

942.  As  the  rainbow  is  produced  by  the  refraction  of  the  sun  s 
rays,  and  every  change  of  position  is  attended  by  a  corresponding 
change  in  the  rays  that  reach  the  eye,  it  follows  that  no  two  persona 
can  see  exactly  the  same  rainbow,  or,  rather,  the  same  appearance 
from  the  same  bow. 

943.  The  Polarization  of  Light  is  a  change  effected  during  reflec- 
tion or  refraction,  by  which  the  etlier  vibrations  on  one  side  of  the 
ray  are  stopped.    (See  par.  1478,  Appendix.)    This  property  of  light 
was  first  discovered  by  Huygens  in  his  investigations  of  the  cause 
of  double  refraction,  as  seen  in  the  Iceland  crystal.     The  attention 
of  the  scientific  world  was  more  particularly  directed  to  it  by  the 
discoveries   of  Malus,  in  1810.     The  knowledge  of  this   singular 
property  of  light  has  afforded  an  explanation  of  several  very  intri- 
cate phenomena  in  Optics,  and  has  afforded  corroborating  evidence 
in  favor  of  the  undulatory  theory ;  but  the  limits  of  this  volume 
will  not  allow  an  extended  notice  of  this  singular  property. 

9-14.  OF  THE  THERMAL,  CHEMICAL,  AND  OTHER  NON-OPTICAL 
EFFECTS  OF  LIGHT.* — The  science  of  Optics  treats  particularly  of 
light  as  the  medium  of  vision.  But  there  are  other  effects  of  this 
agent,  which,  although  more  immediately  connected  with  the  sci- 
ence of  Chemistry,  deserve  to  be  noticed  in  this  connection. 

945.  The  thermal  effects  of  light,  that  is,  its  agency  in  the  excita* 
tion  of  heat  when  it  proceeds  directly  from  the  sun,  are  well  known. 
But  it  is  not  generally  known  that  these  effects  are  extremely  un- 
oqual  in  the  differently  Colored  rays,  as  they  are  refracted  by  the 
prism.  It  has  already  been  stated  that  the  red  rays  appear  to 
possess  the  thermal  properties  in  the  greatest  degree,  and  that  in  the 
other  rays  in  the  spectrum  there  is  a  decrease  of  thermal  power 
towards  the  violet,  where  it  ceases  altogether.  But,  on  the  contrary , 
that  the  chemical  agency  is  the  most  powerful  in  the  vi'olet,  from 
which  it  constantly  decreases  towards  the  red,  where  it  ceases  alto- 
gether. Whether  these  thermal  and  chemical  powers  exist  in  all 
light,  from  whatever  source  it  is  derived,  remains  yet  to  be  ascer- 
tained. The  chromatic  intensity  of  the  colored  spectrum  is  greatest 
in  the  yellow,  from  whence  it  decreases  both  ways,  terminating 
almost  abruptly  in  the  red,  and  decreasing  by  almost  imperceptible 
shades  towards  the  violet,  where  it  becomes  faint,  and  then  wholly 
indistinct.  Thus  it  appears  that  the  greatest  heating  power  resides 
where  the  chemical  power  is  feeblest,  and  the  greatest  chemical 

'*  See  also  pars.  1492-1494. 


OPTICS.  25' 

jw)v\pr  \vht-re  tha  heating  power  is  feeblest,  and  tnat  the  optiooJ 
power  is  the  strongest  between  the  other  two. 

946.  The  chemical  properties  of  light  are  shown  in  this,  that  the 
light  of  the  sun,  and  in  an  inferior  degree  that  of  day  when  the  sun 
is  hidden  from  view,  is  a  means  of  accelerating  chemical  combina- 
tions and  decompositions.  The  following  experiment  exhibits  the 
chemical  effects  of  light  : 

Place  a  mixture  of  equal  parts  (by  measure)  of  chlorine  and  hy- 
drogen gas  in  a  glass  vessel,  and  no  change  will  happen  so  long  as 
the  vessel  be  kept  in  the  dark  and  at  an  ordinary  temperature  ;  but, 
on  exposing  it  to  the  daylight,  the  elements  will  slowly  combine 
and  form  hydrochloric  acid  ;  if  the  glass  be  set  in  the  sun's  rays, 
the  union  will  be  accompanied  with  an  instantaneous  detonation. 
The  report  may  also  be  produced  by  transmitting  ordinary  dayliglif 
through  violet  or  blue  glass  to  the  mixture,  but  by  interposing  a  re^ 
glass  between  the  vessel  and  the  light  all  combination  of  the  elements 
is  prevented. 

947.  The  chemical  effects  of  light  have  recently 
What  is  ,  .  x    .      . 

meant  by  Pho-  "een  employed  to  render  permanent  the  images  ob- 

tograpky.,  or  tained  by  means  of  convex  lenses.  The  art  of  thua 
Heliographytfc^  thera  is  terraed  Photography,  or  Heliography. 
These  words  are  Greek  derivatives ;  the  former  meaning  "  writing 
or  draining  by  means  of  light,"  the  latter  "  writing  or  draw- 
ing by  the  aid  of  the  sun."  (See  par.  1491.) 

I/I/A    •«  4i  948.  The  mode  in  which  the  process  is  performed 

Who  is  the        .  ,.  !-.  c  ,,  m,       r.  ,          c  f      ,  , 

th  f PI  ls  essen*;ia^y  as  follows:  ihe  picture,  formed  by  a 
it  &  h  i  camera  obscura,  is  received  on  a  plate,  the  surface  of 
which  has  been  previously  prepared  so  as  to  make  it 
as  susceptible  as  possible  of  the  chemical  influence  of  light.  After 
the  lapse  of  a  longer  or  shorter  time,  the  light  will  have  so  acted  on 
the  plate  that  the  various  objects  the  images  of  which  were  pro- 
jected upon  it  will  appear,  with  all  their  gradations  of  light  and 
shade,  most  exactly  depicted  in  black  and  white,  no  color  being 

Kesent.  This  is  the  process  commonly  known  by  the  name  of 
iguerreotype,  from  M.  Daguerre,  the  author  of  the  discovery 
Since  his  original  discovery,  he  has  ascertained  that  by  isolating  and 
electrifying  the  plate  it  acquires  such  a  sensibility  to  the  chemical 
influence  of  light  that  one-tenth  of  a  second  is  a  sufficient  time  to 
Dbtain  the  requisite  luminous  impression  for  the  formation  of  the 
picture. 

949.  The  chemical  effects  of  light  are  seen  in  the  varied  colors  of 
the  vegetable  world.  Vegetables  which  grow  in  dark  places  are  either 
<vhite  or  of  a  palish-yellow.  The  sunny  side  of  fruits  is  of  a  richer 
tinge  than  that  which  grows  in  the  shade.  Persons  whose  daily 
employment  keeps  them  much  within  doors  are  pale,  and  more  or 
less  sickly,  in  consequence  of  such  confinement. 


NATURAL    PHILOSOPHY. 

From  what  has  now  been  detailed  with  regard  to  the  nature,  the 
effects,  and  the  importance  of  light,  we  may  see  with  what  reason 
the  great  epic  poet  of  our  language  has  apostrophized  it  in  the 
words 

"  Hail,  holy  Light !  offspring  of  He  aven,  first  born, 
Bright  effluence  sf  bright  essence  increate  ;" 

<i«td  why  the  author  of  the  "Seasons"  has  in  a  similar  manntu 
addressed  it  in  the  terms  : 

"  Prime  cheerer,  Light ! 
Of  all  material  beings  first  aud  best ! 
Efflux  divine  !  Nature's  resplendent  robe  ! 
Without  whose  vesting  beauty  all  were  wrapt 
In  unessential  gloom  ;   and  thou,  0  Sun  ! 
Soul  of  surrounding  worlds,  in  whom  best  seen 
Shines  out  thy  Maker  !  may  I  sing  of  thee  1 " 

950.  ELECTRICITY. —  Electricity   is   the 

What   is   Eleo  .  .  ,       ,.          J         .  . 

name  given  to  an  imponderable  agent  which 

pervades  the  material  world,  and  which  is 
visible  only  in  its  effects. 

951.  It  is  quite  imponderable,  susceptible  of 

high  degrees  Of  intensitv>  with  a  tendency  to 
equilibrium  unlike  tha-l;  of  any   other  known 

agent.     Its  simplest  exhibition  is  seen  in  the  form  of  attraction 

and  repulsion. 

952.  If  a  piece  of  amber,  sealing-wax,  or  smooth  glass,  perfectly 
olean  and  dry,  be  briskly  rubbed  with  a  dry  woollen  cloth,  and  im- 
mediately afterwards  held  over  small  and  light  bodies,  such  as 
pieces  of  paper,  thread,  cork,  straw,  feathers,  or  fragments  of  gold- 
leaf,  strewed  upon  a  table,  these  bodies  will  be  attracted,  and  fly 
towards  the  surface  that  has  been  rubbed,  and  adhere  to  it  for  a 
certain  time. 

953.  The  surfaces  that  have  acquired  this  power  of  attraction 
are  said  to  be  excited;  and  the  substances  thus  susceptible  of  being 
excited  are  called  electrics,  while  those  which  cannot  be  excited  in  a 
similar  manner  are  called  non-electrics. 

954.  The  science  of  Electricity,  therefore, 
What    are    the     ,.    . ,        _.  -,*' 

Metrical  divis-    divides  all  substances  into  two  kinds,  namely, 

ions  of  all  sub-    Electrics,  or  those  suostances  which  can  be 
excited,  and  Non-electrics,  or   those   sub 
stances  which  cannot  be  excited. 


OPTICS.  ^51 

SIOWPI  where  tha  heating  power  is  feeblest,  and  tnut  the  optical 
Bovver  is  the  strongest  between  the  other  two. 

940.  The  chemical  properties  of  light  are  shown  in  this,  that  the 
light  of  the  sun,  and  in  an  inferior  degree  that  of  day  when  the  sun 
is  hidden  from  view,  is  a  means  of  accelerating  chemical  combina- 
tions and  decompositions.  The  following  experiment  exhibits  the 
chemical  effects  of  light : 

Place  a  mixture  of  equal  parts  (by  measure  J  of  chlorine  and  hy- 
drogen gas  in  a  glass  vessel,  and  no  change  will  happen  so  long  as 
the  vessel  be  kept  in  the  dark  and  at  an  ordinary  temperature  ;  but, 
on  exposing  it  to  the  daylight,  the  elements  will  slowly  combine 
and  form  hydrochloric  acid  ;  if  the  glass  be  set  in  the  sun's  rays, 
the  union  will  be  accompanied  with  an  instantaneous  detonation. 
The  report  may  also  be  produced  by  transmitting  ordinary  daylight 
through  violet  or  blue  glass  to  the  mixture,  but  by  interposing  a  re« 
glass  between  the  vessel  and  the  light  all  combination  of  the  elements 
is  prevented. 

947.  The  chemical  effects  of  light  have  recently 
What  is          ,  . 

meant  by  Pho-  been  employed  to  render  permanent  the  images  ob- 

tography.,  or  tained  by  means  of  convex  lenses.  The  art  of  thua 
Heliography?  foin^  them  ig  termed  photograpnVj  or  Heliography. 

These  words  are  Greek  derivatives ;  the  former  meaning  "  writing 
or  drawing  by  means  of  light"  the  latter  "  writing  or  draw- 
ing by  the  aid  of  the  sun."  (See  par.  1491.) 

Who  is  the  ^"  ^he  mO(*e  *n  wn*cn  tne  process  is  performed 

,  f  pi  *s  essentially  as  follows:  The  picture,  formed  by  a 
^o-  °T h  i  l°~  camera  obscura,  is  received  on  a  plate,  the  surface  of 
grap  y .  which  has  been  previously  prepared  so  as  to  make  it 
as  susceptible  as  possible  of  the  chemical  influence  of  light.  After 
the  lapse  of  a  longer  or  shorter  time,  the  light  will  have  so  acted  on 
the  plate  that  the  various  objects  the  images  of  which  were  pro- 
jected upon  it  will  appear,  with  all  their  gradations  of  light  and 
shade,  most  exactly  depicted  in  black  and  white,  no  color  being 
present.  This  is  the  process  commonly  known  by  the  name  of 
Daguerreotype,  from  M.  Daguerre,  the  author  of  the  discovery 
Since  his  original  discovery,  he  has  ascertained  that  by  isolating  and 
electrifying  the  plate  it  acquires  such  a  sensibility  to  the  chemical 
influence  of  light  that  one-tenth  of  a  second  is  a  sufficient  time  to 
obtain  the  requisite  luminous  impression  for  the  formation  of  the 
picture. 

949.  The  chemical  effects  of  light  are  seen  in  the  varied  colors  of 
the  vegetable  world.  Vegetables  which  grow  in  dark  places  are  either 
•vhite  or  of  a  palish-yellow.  The  sunny  side  of  fruits  is  of  a  richer 
tinge  than  that  which  grows  in  the  shade.  Persons  whose  daily 
employment  keeps  them  much  within  doors  are  pale,  and  more  or 
less  aickly,  in  consequence  of  such  confinement. 


260  NATURAL    PHILOSOPHY. 

Tliernno-Ehctricity  ;  4thly,  by  Magnetism.  Frictiona^ 
Electricity  forms  the  subject  of  that  branch  of  Electricitj 
usually  treated  under  the  head  of  Natural  Philosophy; 
Electricity  excited  by  chemical  action  forms  the  subject 
of  Galvanism  ;  and  Electricity  produced  by  the  agency 
of  heat,  or  by  Magnetism,  is  usually  considered  in  connec- 
tion with  the  subject  of  Electro-Magnetism.  The  intimate 
connection  between  these  several  subjects  shows  ho'/r  close 
arc  the  links  of  the  chain  by  which  all  the  departments  of 
physical  science  are  united. 

95U.  The  electric  fluid  is  readily  coinmu- 
by  a  Conductor  n^cate^  fr°m  one  substance  to  another.  Some 
and  a  Non-con-  substances,  however,  will  not  allow  it  to  pass 
tricitl  i  tbrough  or  over  them,  while  others  give  it  a 

free  passage.  Those  substances  through 
which  it  pa?EC&  without  obstruction  are  called  Conductors 
while  those  through  which  it  cannot  readily  pass  are  called 
Non-conduct?)  s  ;  and  it  is  found,  by  experiment,  that  all 
electrics* are  non-conductors,  and  all  non-electrics  ars 
good  conductors  of  electricity. 

960.  The  following  substances  are  electrics,  or  non-conductors 
vF  electricity ;  namely, 

Gntta  Percha. 

Atmospheric  air  (when  dry),  Feathers, 

Glass,  Amber, 

Diamond,  Sulpb.r, 

All  pvccious  stones,  Silk, 

All  *roms  and  resins,  Wool, 

The  r&ides  of  all  metals,  Hair, 

Tiopswax,  Paper, 

Soal  ing-wax,  Cotton. 

All  these  substances  must  be  dry,  or  they  will  beccran  mor* 
<w  less  conductors. 

*  Tbe  terms  "electrics"  and  "  uou -electrics"'  huve  fallec  into  disuse 


ELEU'JKICITY.  26 1 

^61.  The  following  substances  are  non-electrics,  or  conOuctora 

of  electricity  ;   namely, 

All  metals,  Living  animals, 

Charcoal,  Vapor,  or  steam. 

962.  The  following  are  imperfect  conductors  (that  is,  they 
«<unduct  the  electric  fluid,  but  not  so  readily  as  the  substances 
above  mentioned^ ;  namely, 

Water,  Common  wood, 

Green  vegetables,  Dead  animals 

Damp  air,  Bone, 

Wet  wood,  Horn,  &c. 

All  substances  containing  moisture.  • 

When  is  a  con-         963.   When  a  conductor  is  surrounded  on 
tTinsulated?       a11  si(ies  ^J  non-conducting  substances,  it  is 
said  to  be  insulated. 

964.  As  glass  is  a  non-conducting  substance,  any  conducting 
mibstance  surrounded  with  glass,  or  standing  on  a  table  or  stool 
with  glass  legs,  will  be  insulated. 

965.  As  the  air  is  a  non-conductor   when  dry,  a  substance 
which  rests  on  any  non-conducting  substance  will  be  insulated, 
unless  it  communicate  with  the  ground,  the  floor,  a  table,  &c. 

966.  When  a  communication  is  made  be- 
HiuctorSckar^edJ  tween  a  conductor  and  an  excited  surface, 
the  electricity  from  the  excited  surface  is 
Immediately  conveyed  by  the  conductor  to  the  ground ;  but, 
if  the  conductor  be  insulated,  its  whole  surface  will  become 
electrified,  and  it  is  said  to  be  charged. 

What  is  the  967 '.  The    earth   may  be  considered  as  the 

yrand  reservoir    principal  reservoir  of  elec  tricity ;  and  when  a 
communication  exists,  by   means   of  any   con- 
ducting substance,   between   a  body  containing  more  than  its 
natural  share  of  the  fluid  and  the  earth,  the  body  will  imme 
liately  lose  its:  redundant  quantity,  and  the  fluid  will  escape  to 


iJO*2  NATURAL    PHILOSOPHY. 

the  earth.  Thus,  when  a  person  holds  a  metallic  tuoe  to  afc 
excited  surface,  the  electricity  escapes  from  the  surface  30  the 
tube,  and  passes  from  the  tube  through  the  person  to  the  floor  ; 
and  the  floor  being  connected  with  the  earth  by  conducting  sub- 
stances, such  as  the  timbers,  &c.,  which  support  the  building, 
ihe  electricity  will  finally  pass  off,  by  a  regular  succession  of 
conducting  substances,  from  the  excited  surface  to  the  earth. 
But,  if  the  chain  of  conducting  substances  be  interrupted, — that 
is,  if  any  non-conducting  substance  occur  between  the  excited 
surface  and  the  course  which  the  fluid  takes  in  its  progress  to 
the  earth, —  the  conducting  substances  will  be  insulated,  and  be- 
come charged  with  electricity.  Thus,  if  an  excited  surface  be 
connected  by  a  long  chain  to  a  metallic  tube,  and  the  metallic 
tube  be  held  by  a  person  who  is  standing  on  a  stool  with  glass 
legs,  or  on  a  cake  of  sealing-wax,  resin,  or  any  other  non-con- 
ducting substance,  the  electricity  cannot  pass  to  the  ground,  and 
the  person,  the  chain  and  the  tube,  will  all  become  electrified. 

What  is  the  sim-          A,,0     m.        .       .  ,       p  . 

pkst  mode  of        "68.  The  simplest  mode  of  exciting  elec- 

exciting  electric-    tricity  is  by  friction. 

Thus,  if  a  thick  cylinder  of  sealing-wax,  or  sulphur,  or  a 
glass  tube,  be  rubbed  with  a  silk  handkerchief,  a  piece  of  clean 
flannel,  or  the  fur  of  a  quadruped,  the  electric  fluid  will  be 
excited,  and  may  be  communicated  to  other  substances  from  the 
electric  thus  excited. 

Whatever  substance  is  used,  it  must  be  perfectly  dry.  It, 
therefore,  a  glass  tube  be  used,  it  should  previously  be  held  »o 
the  fire,  and  gently  warmed,  in  order  to  remove  all  moisture 
from  its  surface. 

What  is  meant  969'  The  electri<%  excite<*  in  glass  ift 
by  Vitreous  and  called  the  Vitreous  or  positive  electricity  j 

/ridf  T    deC~    and  that  obtained  from  sealing-wax,  or  other 
resinous  substances,  is  called  Resmoust  o/ 
ticgative  electricity 


KLilXJTKICTTY.  2o'b 

970.  The  vitreous  and  lesinous  or,  in 
other  words,  the  positive  and  negative  eleo- 
lofy  is  charged  tricities,  always  accompany  each  other ;  for, 
0/  electricity  f  ^  anj  surface  become  positive,  the  surface 
with  which  it  is  rubbed  will  become  nega- 
tive, and  if  any  surface  be  made  positive,  the  nearest  con- 
ducting surface  will  become  negative ;  and,  if  positive 
electricity  be  communicated  to  one  side  of  an  electric,  (as 
a  pane  of  glass,  or  a  glass  vial),  the  opposite  side  will  be- 
jome  negatively  electrified,  and  the  plate  or  the  glass  if 
then  said  to  be  charged. 

971.  When    one   side   of    a   metallic,    or   other   conductoi 
receives  the  electric  fluid,  its  whole  surface  is  instantly  per- 
vaded ;  but  when  an  electric  is  presented  to  an  electrified  body 
it  becomes  electrified  in  a  small  spot  only. 
What  is  the        972.  When  two  surfaces  oppositely  electrified  are 
effect  when      united,  their  powers  are  destroyed;  and,  if  their 
oppositely       union  be  made  through  the  human  body,   it  pro- 
electrified  are  duces  an  affection  of  the  nerves,  called  an  electric 
united? 


What  is  the  law  of  973-  Similar  states  of  electricity  repe 
electrical  attraction  each  other ;  and  dissimilar  states  attract 
and  repulse!  each  other. 

Thus,  if  two  pith-balls,  suspended  by  a  silk  thread,  are  both 
positively  or  both  negatively  electrified,  they  will  repel  each 
other  ;  but  if  one  be  positively  and  the  other  negatively  electri- 
fied, they  will  attract  each  other. 

What  is  the  974.  The  Leyden  jar  is  a  glass  vessel  used 
Leyden  jar?  for  the  purpose  of  accumulating  the  electric 
tiuid,  procured  from  excited  surfaces. 

Ei plain  97^.  Fig.    143    represents    a    Leyden    jar.      It 

is  a  glass  jar,  coated  both  on  the  inside  and  the 

wutside  with  tin -foil,  with  a  cork,  or  wooden  stopper,  through 


264  NATURAL    1'HILOSOI'lll 

which  a  metallic  rod  passes,  terminating  upwards  in  a  bia 
knob,  and  connected  by  means  of  a  wire,  at  the  other  Fig.  us 
end,  with  the  inside  coating  of  the  jar.  The  coating 
extends  both  on  the  inside  and  outside  only  to  within 
two  or  three  inches  of  the  top  of  the  jar.  Thus  pre- 
pared, when  an  excited  surface  is  applied  to  the 
brass  knob,  or  connected  with  it  by  any  conducting 
surface,  it  parts  with  its  electricity,  the  fluid  enters 
the  jar,  and  the  jar  is  said  to  be  charged. 

When  a  jar  is  976.    When     the    Leyden    jar 
charged  where  h          ,     h    fl   id        contained  Qn  the 
is  the  electric- 
ity? surface   of  the   glass.     The    coating 

serves  only  as  a  conductor  to  the  fluid  ;  and,  as  this  conductor 
within  the  glass  is  insulated,  the  fluid  will  remain  in  the  jar  ujtil 
a  communication  be  made,  by  means  of  some  conducting  sub 
stance,  between  the  inside  and  the  outside  coating  of  the  jar. 
If  then  a  person  apply  one  hand  or  finger  to  the  brass  knob,  and 
the  other  to  the  outside  coating  of  the  jar,  a  communication  will 
be  formed  by  means  of  the  brass  knob  with  the  inside  and  out- 
side of  the  jar,  and  the  jar  will  be  discharged.  A  vial  or  jai 
that  is  insulated  cannot  be  charged. 

What  if  an  Eke-  977.  Anelectrical  battery  is  composed  of 
incai  Battery?  a  num^er  Of  Leyden  jars  connected  together 

The  inner  coatings  of  the  jars  are  connected  together  by 
chains  or  metallic  bars  attached  to  the  brass  knobs  of  each  jar; 
and  the  outer  coatings  have  a  similar  connection  established  by 
placing  the  vials  on  a  sheet  of  tin-foil.  The  whole  battery  may 
then  be  charged  like  a  single  jar.  For  the  sake  of  convenience 
in  discharging  the  battery,  a  knob  connected  with  the  tin-foil  on 
which  the  jars  stand  projects  from  the  bottom  of  the  box  which 
3ontai.is  the  jars. 

What  is  th*  joint-      978.  The  jointed  discharger  is  an  instru- 
ct discharger  ?       ment  used  to  discharge  a  jar  or  battery. 
Explain  Fig.  144  represents  the  jointed    discharger.     It 

riff.  j. 44.     (VQppjgt?  of  two  rods,  generally  of  brass,  terminating 


El.EUTKlUiTY.  2()5 

at  one  ei'd  in  brass  balls,  and  connected  p's-  T44 

together  at  the  other  end  by  a  joint,  like 

that  of  a  pair  of  tongs,  allowing  them 

to  be  opened  or  closed.     It  is  furnished 

with  a  glass  handle,  to  secure  the  person 

who  holds  it  from  the   effects  of  a  shock. 

When  opened,  one  of  the  balls  is  made  to  touch  the  outside 

coating  of  the  jar,  or  the  knob  connected  with  the  bottom  of  the 

battery,  and  the  other  is  applied  to  the  knob  of  the  jar  or  jars. 

A.  communication  being  thus  formed  between  the  inside  and  the 

outside  of  the  jar,  a  discharge  of  the  fluid  will  be  produced. 

Where  must  ^^'  ^ien  a  charge  of  electricity  is  to  be 
i  body  be  sent  through  any  particular  substance,  the 
olaced,  in  or-  gulbgtanco  must  form  a  part  Of  fae  circuit  of 

u€T  10  TCC61VC  A  "^ 

a  charge  of  electricity  ;  that  is,  it  must  be  placed  in  such 
electricity  ?  a  manner  ^hat  the  fluid  cannot  pass  from  the 
inside  to  the  outside  surface  of  the  jar,  or  battery,  without 
passing  through  the  substance  in  its  passage. 

What,  effect  have  sharp      9^0.  Metallic  rods,  with  sharp  points 
metallic  points  ?  silently  attract  the  electric  fluid. 

If  the  balls  be  removed  from  the  jointed  discharger,  and  the 
two  rods  terminate  in  sharp  points,  the  electricity  will  pass  off 
silently,  and  produce  but  little  effect. 

How  may  a  ^^^  981.  A  Ley  den  jar,  or  a  battery,  may  be  silently 
batt  1  '  Discharged  by  presenting  a  metallic  point,  even  that 
lenity  dis-  of  the  finest  needle,  to  the  knob  ;  but  the  point  must 
charged?  ie  ir<night  slowly  towards  the  jar. 

982.  It  is  on  this  principle  that  lightning-rods 
riple  ate  tight-*™  constructed.  The  electric  fluid  is  silently 
ning-rods  drawn  from  the  cloud  by  the  sharp  points  on  the 
c  instructed  ?  rQ^  ^  ^  ^^  prevente(j  from  suddcrily  exploding 

on  high  buildings. 

.  983.  Electricity  of  one  kind  or  the  other  is  gen- 

>nea-/it  by         erally   induced  in  surrounding  bodies  by  the  vie-in- 


NATUltAL    PHILOSOPHY. 

ity  of  a  highly-excited  electric.  This  mode  of  com- 
raunicating  electricity  by  approach  is  styled  induc- 
tion. 

984.  A  body,  on  approaching  another  body  powerfully  elec- 
trified, will  be  thrown  into  a  contrary  state  of  electricity.  Thus, 
a  feather,  brought  near  to  a  glass  tube  excited  by  friction,  will 
be  attracted  to  it ;  and,  therefore,  previously  to  its  touching  the 
tube,  negative  electricity  must  have  been  induced  in  it.  On  the 
contrary,  if  a  feather  be  brought  near  to  excited  sealing-wax,  it 
will  be  attracted,  and,  consequently,  positive  electricity  must 
have  been  induced  in  it  before  contact. 

What  is  985-  When  electricity  is  communicated  from 

Electricity  by  one  body  to  another  in  contact  with  it,  it  is 
Transfer*  ^^  electricity  by  transfer. 

W/,attsan       986    The  eiectricai    machine   is   a   machine 

Electrical 

Maeki we,  and  constructed  for  the  purpose  of  accumulating  or 

Ir^it'con  collectiaS  electricity,  and  transferring  it  to  other 
structed?  substances. 

987.  Electrical  Machines  are  made  in  various  forms,  but  all 
on  the  same  principle,  namely,  the  attraction  of  metallic  points. 
The  electricity  is  excited  by  the  friction  of  silk  on  a  glass  sur- 
face, assisted  by  a  mixture  or  preparation  called  an  amalgam, 
composed  of  mercury,  tin,  and  zinc.  That  recommended  by 
Singer  is  made  by  melting  together  one  ounce  of  tin  and  two 
ounces  of  zinc,  which  are  to  be  mixed,  while  fluid,  with  six 
ounces  of  mercury,  and  agitated  in  an  iron  or  thick  wrooden  box, 
until  cold.  It  is  then  to  be  reduced  to  a  very  fine  powder  in  a 
mortar,  and  mixed  with  a  sufficient  quantity  of  lard  to  form  it 
into  a  paste. 

The  glass  surface  is  macTe  either  in  the  form  of  a  cylinder  or 
a  circular  plate,  and  the  machine  is  called  a  cylinder  or  a  plata 
maohine,  according  as  it  is  made  with  a  cylinder  or  with  a  plate. 
Explain  988.  Fig,  145  represents  a  plate  electrical  m.v 

Fig.  145.  chine.  A  D  is  the  stand  of  the  machine,  L  L  L  L 


207 


are  the  four  glass  legs,  or  posts,  which  support  and  insulate  the 
parts  of  the  machine.  P  is  the  glass  plate  (which  in  some  ma- 
chines is  a  hollow  cylinder)  from  wh;ch  the  electricity  is  excited, 
and  H  is  the  handle  by  which  the  plate  (or  cylinder)  is  turned. 
R  is  a  leather  cushion,  or  rubber,  held  closely  to  both  sides  of 
the  glass  plate  by  a  brass  clasp,  supported  by  the  post  G  L 
which  is  called  the  rubber-post.  S  is  a  silk  bag,  embraced  by 
the  same  clasp  that  holds  the  leather  cushion  or  rubber ;  and  it 
is  connected  by  strings  S  S  S  attached  to  its  three  other  corners, 
and  to  the  legs  L  L  and  the  fork  F  of  the  prime  conductor.  G 
IP  the  prime  conductor,  terminating  at  one  end  with  a  movable 

Fig.  145. 


BO — 


brass  ball.  B,  and  at  the  other  by  the  fork  F,  which  has  one 
prong  on  each  side  of  the  glass  plate.  On  each  prong  of  the 
fork  there  are  several  sharp  points  projecting  towards  the  plate, 
to  collect  the  electricity  as  it  is  generated  by  the  friction  of  the 
plate  against  the  rubber.  V  is  a  chain  or  wire,  attached  to  the 
brass  ball  on  the  rubber-post,  and  resting  on  the  table  or  the 
fioor,  designed  to  convey  the  fluid  from  the  ground  to  the  plate 
When  negative  electricity  is  to  be  obtained,  this  chain  is  re 
moved  from  the  rubber-post  and  attached  to  the  prime  conductor 
and  the  electricity  is  to  be  gathered  from  the  ball  on  the  rubber 
post. 

Explain  the       ^89.  OPERATION  OF  THE  MADHINE.  —By  turning 
operation  of  the  handle  H,  the  glass  plate  is  pressed  by  the  rub- 


Ii68  NATURAL    PHILOSOPHY. 

the  Electri-  ber.  The  friction  of  the  rubber  against  the  glas* 
cal  Machine.  pla^  e  (or  cylinder)  produces  a  transfer  of  the  elec- 
tric fluid  from  the  rubber  to  the  plate;  that  is,  the  cushion  be- 
comes negatively  and  the  glass  positively  electrified.  The  fluid 
which  thus  adheres  to  the  glass,  is  carried  round  by  the  revolu- 
tion of  the  cylinder ;  and,  its  escape  being  prevented  by  the  silk 
oag,  or  flap,  which  covers  the  plate  (or  cylinder)  until  it  comes 
to  the  immediate  vicinity  of  the  metallic  points  on  the  fork  F, 
it  is  attracted  by  the  points,  and  carried  by  them  to  the  prime 
conductor.  Positive  electricity  is  thus  accumulated  on  the  prime 
conductor,  while  the  conductor  on  the  rubber-post,  being  deprived 
of  this  electricity,  is  negatively  electrified.  The  fluid  may  then 
be  collected  by  a  Leyden  jar  from  the  prime  conductor,  or  con- 
veyed, by  means  of  a  chain  attached  to  the  prime  conductor,  to 
any  substance  which  is  to  be  electrified.  If  both  of  the  conduc- 
tors be  insulated,  but  a  small  portion  of  the  electric  fluid  can  be 
excited ;  for  this  reason,  the  chain  must  in  all  cases  be  attached 
to  the  rubier-post,  when  positive  electricity  is  required,  and  to 
the  prime  conductor  when  negative  electricity  is  wanted. 

What  is  an  ^90.  ^n  ^e  Pr^me  conductor  is  placed  an 
Electron.-  Electrometer,  or  measurer  of  electricity.  It  ia 
*what*mincl  ma(^e  ^n  various  forms,  but  always  on  the  prin- 
ple  is  it  con-  ciple  that  similar  states  of  electricity  repel  each 
•</rucied-  other. 

It  sometimes  consists  of-  a  single  pith-ball,  attached  to  a  light 
rod  in  the  manner  of  a  pendulum,  and  behind  is  a  graduated  arc, 
or  circle,  to  measure  the  repulsive  force  by  degrees.  Sometimes 
it  is  more  simply  made  (as  in  the  figure),  consisting  of  a  wooden 
ball  mounted  on  a  metallic  stick,  or  wire,  having  two  pith-balls, 
suspended  by  silk,  hair,  or  lineif  threads.  When  the  machine 
is  worked,  the  pith-balls,  being  both  similarly  electrified,  repel 
each  other  ;  and  this  caus  is  them  to  fly  apart,  as  is  represented 
in  the  figure;-  and  they  will  continue  elevated  until  the  electric- 
ity is  drawn  off.  But,  if  an  uninsulated  conducting  substance 
tou".h  the  prime  conductor,  the  pith-balls  will  fall.  The  height 


KLECT1CIU1TY.  2(59 

k  which  the  balls  rise,  and  the  quickness  with  which  they  are 
elevated,  afford  some  test  of  the  power  of  the  machine.  This 
simple  apparatus  may  be  attached  to  any  body  the  electricity 
of  which  we  wish  to  measure. 

The  balls  of  the  electrometer,  when  elevated,  are  attracted  by 
any  resinous  substance,  and  repelled  by  any  vitreous  substance 
that  has  been  previously  excited  by  friction. 

991.  If  an  electric,  or  a  non-conductor,  be  presented  to  the  prime 
conductor,  when  charged,  it  will  produce  no  effect  on  the  balls ; 
but  if  a  non-electric,  or  any  conducting  substance,  be  presented 
to  the  conductor,  the  balls  of  the  electrometer  will  fall.  This 
shows  that  the  conductor  has  parted  with  its  electricity,  and 
that  the  fluid  has  passed  off  to  the  earth  through  the  substance, 
and  the  hand  of  the  person  presenting  it. 

~       .,  992.  An  Electroscope  is  an  instrument,  of  more 

Bennett's  delicate  construction,  to  detect  the  presence  of 
Electroscope,  electricity.  The  most  sensitive  of  this  kind  of 
apparatus  is  that  called  Bennett's  Gold-leaf  Electroscope,  im- 
proved by  Singer.  It  consists  of  two  strips  of  gold-leaf  suspended 
under  a  glass  covering,  which  completely  insulates  them.  Strip? 
of  tin-foil  are  attached  to  the  sides  of  the  glass,  opposite  the 
gold-leaf,  and  when  the  strips  of  gold-leaf  diverge,  they  will  touch 
the  tin-foil,  and  be  discharged.  A  pointed  wire  surmounts  the 
instrument,  by  which  the  electricity  of  the  atmosphere  may  be 
observed. 

993.  An  Electrophorus  is  a  simple  apparatus  by  which  small 
portions  of  electricity  may  be  generated  by  induction.  •  It  con- 
sists of  a  disc,  or  circular  cake  of  resinous  substance,^  on  which 
is  laid  a  smaller  circular  disc  of  metal,  with  a  glass  handle.  Rub 
the  resinous  disc  with  hair  or  the  fur  of  some  animal,  and  the 
metallic  disc,  being  pressed  down  on  the  resii  by  the  finger, 
may  then  be  raised  by  the  glass  handle.  It  will  contain  a  small 
portion  of  electricity,  which  may  be  communicated  to  the  Leyden 
jar,  and  thus  the  jar  may  slowly  be  charged. 

«  A  mixture  of  Shell-lac   resin  and  Venice- turpentine,  aast  in  a  tin  mcuH 


Z70  NATURAL    PHILOSOPHY. 

994.  EXPERIMENTS    WITH   THE   ELECTRICAL   MACHINE  —  In 
peforming  experiments  with  the  Electrical  Machine,  great  ear* 
inurt  be  taken  that  all  its  parts  be  perfectly   dry  and  clean 
Moisture  arid  dust,  by  carrying  off  the  electricity  as  fast  as  it  is 
generated,  prevent  successful  action.     Clear  and  cold  weather 
should  be  chosen,  if  possible,  as  the  machine  will  always  perform 
its  work  better  then. 

995.  When  the  machine  is  turned,  if  a  person  touch  the  prime 
conductor,  the  fluid  passes  off  through  the  person  to  the  floor 
without  his  feeling  it.     But  if  he  present  his  finger,  his  knuckle, 
or  any  part  of  the  body,  near  to  the  conductor,  without  touching 
it,  a  spark  will  pass  from  the  conductor  to  the  knuckle,  which 
will  produce  a  sensation  similar  to  the  pricking  of  a  pin  or 
needle. 

996.  If  a  person  stand  on  a  stool  with  glass  legs,  or  any  other 
non-conductor,  he  will  be  insulated.     If  in  this  situation   he 
touch  the  prime  conductor,  or  a  chain  connected  with  it,  when 
the  machine  is  worked,  sparks  may  be  drawn  from  any  part  of 
the  body  in  the  same  manner  as  from  the  prime  conductor. 
While  the  person  remains  insulated,  he  experiences  no  sensation 
from  being  filled  with  electricity ;  or,  if  a  metallic  point  be  pre- 
sented to  any  part  of  his  body,  the  fluid  may  be  drawn  off 
silently,  without  being  perceived.     But  if  he  touch  a  blunt  piece 
of  metal,  or  any  other  conducting  substance,  or  if  he  step  from 
the  stool  to  the  floor,  he  will  feel  the  electric  shock  ;  and  the 
shock  will  vary  in  force  according  to  the  quantity  of  fluid  with 
which  he  is  charged. 

997.  THE  TISSUE  FIGURE.     Fig.  146  is  a 
dgure  with  a  dress  of  fancy  paper  cut  into 
narrow  strips.     When  placed  on  the  prime 
conductor,  or,  being  insulated,  is  connected 
with  it,  the  strips  being  all  electrified  will 
recede  and  form  a  sphere  around  the  head. 
On  presenting  a  metallic  point  to  the  elec- 
trified strips,  very  singular  combinations 
will  take  place.     If  the  electrometer  be 


ELECTRICITY.  271 

removed  from  the  prime  conductor,  and  a  tuft  of  feathers,  01 
hair,  fastened  to  a  stick  or  wire,  be  put  in  its  place,  on  turning 
the  machine  the  feathers  or  hair  will  become  electrified,  and  the 
separate  hairs  will  rise  and  repel  each  other.  A  toy  is  in  this 
way  constructed,  representing  a  person  under  excessive  fright. 
On  touching  the  head  with  the  hand,  or  any  conducting  substance 
not  insulated,  the  hair  will  fall. 

How  is  the  998.  The  Leyden  jar  may  be  charged  by  pre- 
Leydenjar  genting  it  to  ^he  prime  conductor  when  the  machine 
is  worked.  If  the  ball  of  the  jar  touch  the  prime 
conductor  it  will  receive  the  fluid  silently  ;  but,  if  the  ball  of 
the  jar  be  held  at  a  small  distance  from  the  prime  conductor,  the 
sparks  will  be  seen  darting  from  the  prime  conductor  to  the  jar 
with  considerable  noise. 

999.  The  jar  may  in  like  manner  be  filled  with  negative  elec- 
tricity by  applying  it  to  the  ball  on  the  rubber-post,  and  con- 
necting the  chain  with  the  prime  conductor. 

1000.  If  the  Leyden  jar  be  charged  from  the  prime  conductor 
(that  is,  with  positive  electricity),  and  presented  to  the  pith-balls 
of  the  electrometer,  they  will  be  repelled ;  but  if  the  jar  be 
charged  from  the  brass  ball  of  the   rubber-post  (that  is,  with 
negative  electricity),  they  will  be  attracted. 

1001.  If  the  ball  of  the  prime  conductor  be  removed,  and  a 
pointed  wire  be  put  in  its  place,  the  current  of  electricity  flowing 
from  the  point  when  the  machine  is  turned  may  be  perceived  by 
placing  a  lighted  lamp  before  it ;  the  flame  will  be  blown  from 
the  point ;  and  this  will  be  the  case  in  what  part  soever  of  the 
machine  the  point  is  placed,  whether  on  the  prime  conductor  or 
the  rubber  ;  or  if  the  point  be  held  in  the  hand,  and  the  flame 
placed  between   it  and  the  machine,  thus  showing  that  in  all 
cases  the  fluid  is  blown  from  the  point.     Delicate  apparatus 
may  be  put  in  motion  by  the  electric  fl  uid  when  issuing  from  a 
point.     In  this  way  electrical  orreries,  mills,  &c.,  are  constructed. 

1002.  If  the  electrometer  be  removed  from  the  prime  con- 


NATURAL    PHILOSOPHY. 


Fig.  147. 


ductor,  and  a  pointed  wire  be  substituted  for  /t,  a  wire  with 
sharp  points  bent  in  the  form  of  an  S,  balanced  on  it,  will  ba 
made  to  revolve  rapidly.  In  a  similar  manner  the  motion  of 
the  sun  and  the  earth  around  their  common  centre  of  gravity, 
together  with  the  motion  of  the  earth  and  the  moon,  may  be 
represented.  This  apparatus  is  sometimes  called  an  Electrical 
Tellurium.  It  may  rest  on  the  prime  conductor  or  upon  an  insu- 
lated stand. 

Describe  1003.  A  chime  of  small  bells  on  a  stand, 
Fig.  147.  Fig.  147,  may  also  be  rung  by  means  of 
brass  balls  suspended  from  the  revolving  wires. 
The  principle  of  this  revolution  is  similar  to  that 
mentioned  in  connection  with  the  revolving  jet, 
j?1ig.  98,  which  is  founded  on  the  law  that  action 
and  reaction  are  equal  and  in  opposite  directions. 

1004.  If  powdered  resin  be  scattered  over 
cotton-wool,  loosely  wrapped  on   one   end  of  the 

;ointed  discharger,  it  may  be  inflamed  by  the  discharge  of  the 
battery  or  a  Leyden  jar.  Gunpowder  may  be  substituted  for  thf 
resin. 

1005.  The  universal  discharger  is  an   instrument  for 
directing  a  charge  of  electricity  through  any  substance, 
with  certainty  and  precision. 

Explain  1006.  It  consists  of  two  sliding  rods,  A  B  and  C 

tg.  14».  j^  terminating  at  the  extremities,  A  and  B,  with  brass 
balls,  and  at  the  other  ends  which 
rest  upon  the  ivory  table  or  stand 
E,  having  a  fork,  to  which  any 
small  substance  may  be  attached. 
The  whole  is  insulated  by  glass 
legs,  or  pillars.  The  rods  slide 
through  collars,  by  which  means  their  distance  from  one  anoth*  r 
may  be  adjusted. 

1007.  In  using  the  universal  discharger  one  of  the  rods   01 
Slides  must  be  connected  by  a  chain,  or  otherwise,  with  the  out 


Fig.  148. 


ELECTRICITY*  JJY'j 

mde,  and  the  other  with  the  inside  coating  of  ftie  jar  or  battery. 
By  this  means  the  substance  through  which  tke  charge  is  to  b<i 
sent  is  placed  within  the  electric  circuit. 

1008.  By  means  of  the  universal  discharger,  any  small  metal- 
lic substance  may  be  burnt.     The  substance  must  be  placed  in 
the  forks  of  the  slides,  and  the  slides  placed  within  the  electric 
circuit,  in  the  manner  described  in  the  last  paragraph.     In  the 
san.6  manner,  by  bringing  the  forks  on  the  slides  into  contact 
with  a  substance  placed  upon  the  ivory  stand  of  the  discharger, 
such  as  an  egg,  a  piece  of  a  potato,  water,  £c.,  it  may  be  illu 
minated. 

1009.  Ether  or  alcohol  may  be  inflamed  by  a  spark  communi- 
cated from  a  person,  in  the  following  manner  :  The  person  stand- 
ing on  the  insulating  stool  receives  the  electric  fluid  from  the 
prime  conductor  by  touching  the  conductor  or  any  conducting 
substance  in   contact  with  it;  he  then  inserts  the  knuckles  of 
his  hand  in  a  small  quantity  of  sulphuric  ether,  or  alcohol,  held 
in  a  shallow  metallic  cup,  by  another  person,  who  is  not  insu- 
lated, and  the  ether  or   alcohol  immediately  inflames.     In  this 
case  the  fluid  passes  from  the  conductor  to  the  person  who  is 
insulated,  and  he  becomes  charged  with  electricity.     As  soon 
as  he  touches  the  liquid  in  the  cup,  the  electric  fluid,  passing  from 
him  to  the  spirit,  sets  it  on  fire. 

1010.  The  electrical  bells  are  designed  to  show  the  effects 
of  electrical  attraction  and  repulsion. 

1011.  In  some  sets  of  instruments,  the  bells  are  insulated  on  a 
separate  stani  ;   but  the  mode  here  described  is  a  convenient  mode 
of  connecting  them  with  the  prime  conductor. 

1012.  They  are  Fig.  149. 


Fig. 


plied:  The  ball 
B  of  the  prime  conductor,  with 
its  rod,  is  to  be  unscrewed,  and 
the  rod  on  which  the  bells  are 
suspended  is  to  be  screwed  in  its 


1 


'274  NATURAL    PHILOSOPHY 

place.  The  middle  bell  is  to  be  connected  by  a  chain  with 
the  table  or  the  floor.  When  the  machine  is  turned,  the  balls 
suspended  between  the  bells  will  be  alternately  attracted  and 
repelled  by  the  bells,  and  cause  a  constant  ringing.  If  the  bat- 
tery be  charged,  and  connected  with  the  prime  conductor,  the 
bells  will  continue  to  ring  until  all  the  fluid  from  the  battery 
has  escaped. 

It  may  be  observed,  that  the  fluid  from  the  prime  conductor 
passes  readily  from  the  two  outer  bells,  which  are  suspended  by 
chains;  they,  therefore,  attract  the  two  balls  towards  them 
The  balls,  becoming  electrified  by  contact  with  the  outer  bells, 
are  repelled  by  them,  and  driven  to  the  middle  bell,  to  which 
they  communicate  their  electricity  ;  having  parted  with  their 
electricity,  they  are  repelled  by  the  middle  bell,  and  again 
attracted  by  the  outer  ones,  and  thus  a  constant  ringing  is 
maintained.  The  fluid  which  is  communicated  to  the  middle 
bell,  is  conducted  to  the  earth  by  the  chain  attached  to  it. 

Explain  what  10*3-  SPIRAL  TUBE.—  The  passage  of  the 
Fig.  150  rep-  electric  fluid  from  one  conducting  substance  to 
resents.  another,  is  beautifully  exhibited  by  means  of  a 

^lass  tube,  having  a  brass   ball  at  each   end,  and   coated    in 

Fig.  150. 


\he  inside  with  small  pieces  of  tin-foil,  placed  at  small  dis- 
tances from  each  other  in  a  spiral  direction,  as  represented  in 
F?g.  150. 

1014.  In  the  same  manner  various  figures,  letters  and  words,  may 
be  represented,  by  arranging  similar  pieces  of  tin-foil  between  two 
pieces  of  flat  glass.  These  axperiments  appear  more  brilliant  in  a 
darkened  room. 

1015.  THE  HYDROGEN  PISTOL.  —  The  hydrogen 
ig.       pist0l  is  made  in  a  variety  of  forms,  sometime? 
ID  the  exact  form  of  a  pistol  and  sometime?  iu 


ELECTEICITT. 


276 


Fig    151 


Explain    Fig. 
152. 


Fig.  152. 


the  form  of  a  piece  of  ordnance.  The  form  in 
Fig.  151  is  a  simple  and  cheap  contrivance,  and 
is  sufficient  to  explain  the  manner  in  which  the 
instrument  is  to  be  used  in  any  of  its  forms. 
It  is  to  be  filled  with  a  mixture  of  hydrogen 
and  oxygen,  or  hydrogen  and  air.  When 
thus  prepared,  if  the  insulated  knob  K  be  pre- 
sented to  the  prime  conductor,  it  wUl  immediately  explode. 

1016.  A   very   convenient   and    economicai 
way  of  procuring  hydrogen  gas  for  this  and 
other  experiments,  is  by  means  of  the  hydrogen 

%as  gejierator,  as  represented  in  Fig.  152,  It  consists  of  a  gla&* 
vessel,  with  a  brass  cover,  in  the  centre  of  which  is 
a  stop-cock;  from  the  inside  of  the  cover  another 
glass  vessel  is  suspended,  with  its  open  end  down- 
wards. Within  this  a  piece  of  zinc  is  suspended  by 
*  wire.  The  outer  vessel  contains  a  mixture  of  sul- 
phuric acid  and  water,  about  nine  parts  of  water  to 
one  of  acid.  When  the  cover,  to  which  the  inner 
glass  is  firmly  fix*ed,  is  placed  upon  the  vessel,  the 
acid,  acting  upon  the  zinc,  causes  the  metal  to 
absorb  the  oxygen  of  the  water,  and  the  hydrogen, 
the  other  constituent  part  of  the  water,  being  thus 
disengaged,  rises  in  the  inner  glass,  from  which  it  expels  the 
water ;  and  when  the  stop-cock  is  turned  the  hydrogen  gas  may 
be  collected  in  the  hydrogen  pistol,  or  any  other  vessel.  In  the 
use  of  hydrogen  gas  for  explosion,  it  will  be  necessary  to  dilute 
the  gas  with  two  or  three  times  as  much  air. 

1017.  ELECTRICAL    SPORTSMAN.  —  Fig.    158 
Describe  the  Elec-  represents  the  Electrical  Sportsman,    From  tht 
Incal  Sportsman. 

larger  ball  of  a  Leyden  jar  two  birds,  made  of 

pith  (a  substance  procured  in  large  quantities  from  the  corn- 
stalk, the  whole  of  which,  except  the  outside,  is  composed  of 
ptfk\  are  suspended  by  a  linesn  thread,  silk,  or  hair.  When  the 
jar  is  charged,  the  birds  wil  rise,  as  represented  iu  the  figure, 
12 


276 


NATURAL    PHILOSOPHY. 


on  account  of   the    repul- 
sion of  the  fluid  in  the  jar. 

1018.  If  the  jar  be  then 
placed  on  the  tin-foil  of  the 
stand,  and  the  smaller  ball 
placed  within  a  half  inch 
of  the  end  of  the  gun,  a 
discharge  will  be  produced, 
and  the  birds  will  fall. 


Kg 


Explain   Fig. 
154. 


Fig.  154. 


n 


1019.  If  images,  made  of  pith,  or  small 
pieces  of  paper,  are  placed  ut,der  the  insulated 
stool,  and  a  connection  be  made  between  the 
prime  conductor  and  the  top  of  the  stool,  the  images  will  be 
alternately  attracted  and  repelled ;  or,  in  other  words,  they  wii> 
first  rise  to  the  electrified  top  of  the  stool,  and  thus  becoming 
themselves  electrified,  will  be  repelled,  and  fall  to  the  ground, 
the  floor,  or  the  table  ;  where,  parting  with  their 
electricity,  they  will  again  be  attracted  by  the 
stool,  thus  rising  and  falling  with  considerable 
rapidity.  In  order  to  conduct  this, experiment 
successfully,  the  images,  &c.,  must  be  placed 
within  a  short  distance  of  the  bottom  of  the 
stool. 

1020.  On   the    same  principle    light   figures 
may  be  made  to  dance  when  placed  between  two 
discs,  the  lower  one  being  placed  upon  a  sliding 
stand  with  a  screw  to  adjust  the  distance,  and 

the  upper  one  being  suspended  from  the  prime  conductor,  as  in 
Fig.  154. 

1021.  A  hole  may  be  perforated  through  a  quire  of  paper, 
by  charging  the  battery,  resting  the  paper  upon  the  briss  ball 
of  the  battery,  and  making  a  communication,  by  means  of  the 
jointed  discharger,  between  the  ball  of  one  of  the  jars,  and  the 
brass  ball  of  the  box.     The  paper,  in  this  case,  will  be  between 
the  ball  of  the  battery  and  the  end  of  the  discharger. 


KLECTK1UITY.  277 

1022.  Gold-leaf  may  be  forced  into   the  pores  of  glass  by 
placing  it  between  two  slips  of  window-glass,  pressing  the  slips 
of  glass  firmly  together,  and  sending  a  shock  from   a  battery 
through  tdem. 

If  gold-leaf  be  placed  between  two  cards,  and  a  strong  charge 
be  passed  through  them,  it  "will  be  completely  fused. 

1023.  When  electricity  enters  at  a  point,  it  appears  in 
the  form  of  a  star ;  but  when  it  goes  out  from  a  point,  it 
puts  on  the  appearance  of  a  brush. 

1024.  The  thunder-house,  Fig.  155,  is  de- 
/)escn£ei  Fig.  signe<j  to  show  the  security  afforded  by  light- 
ning-rods when  lightning  strikes  a  building 
This  is  done  by  placing  a  highly-combustible  material  in  th» 
inside  of  the  house,  and  passing  a 
charge  of  electricity  through  it.  On 
the  floor  of  the  house  is  a  surface  of 
tin-foil.  The  hydrogen  pistol,  being 
filled  with  hydrogen  gas  from  the 
gasometer,  must  be  placed  on  the  floor 
of  the  thunder-house,  and  connected 
with  the  wire  on  the  opposite  side. 
The  house  being  then  put  together,  a  chain  must  be  connected 
with  the  wire  on  the  side  opposite  to  the  lightning-rod,  and  the 
other  end  placed  in  contact  either  with  a  single  Leyden  jar  or 
with  the  battery.  When  the  jar,  thus  situated,  is  charged,  if  a 
connection  be  formed  between  the  jar  and  the  points  of  the 
lightning-rod,  the  fluid  will  pass  off  silently,  and  produce  no 
effect.  But,  if  a  small  brass  ball  be  placed  on  the  points  of  the 
rod,  and  a  charge  of  ele^tncity  be  sent  to  it  from  the  jar  or 
the  battery,  the  gas  in  the  pistol  will  explode,  and  throw  the 
parts  of  the  house  asunder  with  a  loud  noise. 

1025.  The  success  of  this  experiment  depends  upon  the  proper  con- 
nection of  the  jar  with  the  lightning-rod  and  the  electrical  pistol. 
On  the  side  of  the  house  opposite  to  the  lightning-rod  there  is  a 
wire,  passing  thrrugh  the  side,  and  terminat?iig  on  the  outside  in  a 


278  NATURAL   x-HILOSOPilY. 

hook  ^  'hen  the  house  is  put  together,  this  wire,  ir  the  inside- 
must  touch  the  tin- foil  on  the  floor  of  the  house.  The  hydrogen 
pistol  must  stand  on  the  tin-foil,  and  its  insulated  knob,  or  wire,  pro- 
jecting from  its  side,  must  be  connected  with  the  lower  end  of  the 
lightning-rod,  extending  into  the  inside  of  the  house.  A  communi- 
cation must  then  be  made  between  the  hook  on  the  outside  of  the 
house  and  the  outside  of  the  jav,  or  battery.  This  is  conveniently 
done  by  attaching  one  end  of  a  chain  to  the  hook,  and  holding  the 
other  end  in  the  hand  against  the  side  of  a  charged  jar.  By  pre- 
senting the  knob  of  the  jar  to  the  points  of  the  lightning- rod  no 
effect  is  produced  ;  but  if  a  brass  ball  be  placed  on  the  points  at  P, 
and  the  knob  of  the  jar  be  presented  to  the  ball,  the  explosion  will 
take  place.  If  the  charged  jar  be  very  suddenly  presented  to  the 
points,  the  explosion  may  take  place ;  and  the  jar  may  be  silently 
discharged  if  it  be  brought  very  slowly  to  the  ball.  The  thunder- 
house  is  sometimes  put  together  with  magnets. 

What  is  light-  1026.  The  phenomena  of  lightning  are 
ning  and  thun-  caused  by  the  rapid  motion  a;  vast  quanti- 
ties of  electric  matter.  Thunder  is  the  noise 
which  accompanies  the  passage  of  electricity  through  the 
air. 

What  is  sup-  1027.  The  aurora  borealis  (or  northern 
posed  to  be  the  lights)  is  supposed  to  be  caused  by  the  electric 

cause  of  the  fluid  passing  through  highly-rarefied  air ;  and 

northern  lights?  9.  J 

most  of  the  great  convulsions  of  nature,  suca 

as  earthquakes,  whirlwinds,  hurricanes,  water-spouts,  &c.,  are 
generally  accompanied  by  electricity,  and  often  depend  upon  it 

1028.  The  electricity  which  a  body  manifests  by  being  brought 
near  to  an.  excited  body,  without  receiving  a  spark  from  it,  is 
said  to  be  acquired  by  induction.     When  an  insulated  but  un. 
electrified  conductor  is  brought  near  an  insulated  charged  con- 
ductor, the  end  near  to  the  excited  conductor  assumes  a  state 
of  opposite  electricity,  while  the  farther  end  assumes  the  same 
kind   of    electricity, —  that  is,  if  the  conductor  be   electrified 
positively,  the  unelectrified  conductor  will  be  negative  at  the 
nearer  end,  and  positive  at  the  further  end,  while  the  middle 
point  evinces  neither  positive  nor   negative   electricity.     [See 
No.  993. 

1029.  The  experiments  which  have  now  been  described  exem- 


ELKOTKIOITY. 

all  the  elementary  principles  of  the   science   of  electricity 
hese  experiments  may  be  varied,  multiplied,  and  extended  in  innu- 
merable forms,  by  an  ingenious  practical  electrician.     Among  other 
things  with  which  the  subject  may  be  made  interesting,  may  be 
mentioned  the  following  facts,  &c. 

1030  A  number  of  feathers,  suspended  by  strings  from  an  insu- 
lated conducting  substance,  will  rise  and  present  the  appearance  of 
a  flight  of  birds.  As  soon  as  the  substance  is  discharged,  the 
feathers  will  fall.  The  experiment  may  be  varied  by  placing  the 
sportsman  on  the  prime  conductor,  without  the  use  of  the  Leyden 
jar,  to  which  the  birds  are  attached. 

1031.  Instead  of  the  Leyden  jar,  a  plate  of  common  glass  (a  pane 
of  window-glass,  for  instance)   may  be  coated  on  both  sides  with 
tin- foil,  leaving  the  edges  bare.     A  bent  wire  balanced  on  the  edge 
of  the  glass,  to  the  ends  of  which  balls  may  be  attached,  with  an 
image  at  each  end,  may  be  made  to  represent  two  persons  tilting,  on 
the  same  principle  by  which  the  electrical  bells  are  made  to  ring. 

1032.  Miniature  machinery  has  been  constructed,  in  which  the 
power  was  a  wheel,  with  balls  at  the  ends  of  the  spokes,  situated 
within  the  attractive  influence  of  two  larger  balls,  differently  electri- 
fied. As  the  balls  on  the  spokes  were  attracted  by  one  of  the  larger 
balls,  they  changed  their  electrical  state,  and  were  attracted  by  the 
otb^r,  which,  in  its  return,  repelled  them,  and  thus  the  motion  being 
ghen  to  the  wheel  was  communicated  by  cranks  at  the  end  of  the 
axle  to  the  saws  above. 

1033.  When  the   hand  is  presented  to  the   prime   conductor,  a 
spark  is  communicated,  attended  with  a  slightly  painful  sensation. 
But,  if  a  pin  or  a  needle  be  held  in  the  hand  with  the  point  towards 
the  conductor,  neither  spark  nor  pain  will  be  perceived,  owing  tc 
the  attracting  (or,  perhaps,  more  properly  speaking,  the  receiving.) 
power  of  the  point. 

1034.  That  square  rods  are  better  than  round  ones  to  conduct 
electricity  silently  to  the  ground,  and  thus  to  protect  buildings; 
may  be  proved  by  causing  each  kind  of  rod  to  approach  the 
prime  conductor  when  charged.     It  will  thus  be  perceived  that, 
while  little  effect  is  produced  on  the  pith-balls  of  the  electrom- 
eter by  the  near  approach  of  the  round  rod,  on  the  approach 
of  the  square  one  the  balls  will  immediately  fall.     The  round 
rr.d,  also,  will  produce  an  explosion  and  a  spark  from  the  ball 
r.f  the  prime  conductor,  while  the  square  one  will  draw  off  the 
fluid  silently. 

103.^.  The  effects  of  pointed  conductors  upon  clouds  charged 
with  electricity  may  be  familiarly  exemplified  by  suspending  a 
small  fleece  of  cotton-wool  from  the  prime  conductor,  and 


280  NATURAL    rillLOSOPilY. 

other  smaller  fleeces  from  the  upper  one,  by  small  filaments. 
On  presenting  a  point  to  them  they  will  be  repelled,  and  all 
drawn  together ;  but,  if  a  blunt  conductor  approach  them,  they 
will  be  attracted. 

1036.  From  a  great  variety  of  facts,  it  has  been  ascertained, 
that  lightning-rods  afford  but  little  security  to  any  part  of  a 
building  beyond  twenty  feet  from  them ;  and  that  when  a  rod  is 
painted  it  loses  its  conducting  power. 

What  are  the  1037.  The  lightning-rods  of  the  most  ap- 
best  kinds  of  proved  construction,  and  in  strictest  accordance 
with  philosophical  principles,  are  composed  of 
*maR  square  rods,  similar  to  nail-rods.  They  run  over  the 
building,  and  down  each  of  the  corners,  presenting  many 
elevated  points  in  their  course.  At  each  of  the  corners,  and  on 
the  chimneys,  the  rods  should  be  elevated  several  feet  above  the 
building.  If  the  rods  are  twisted,  it  will  be  an  improvement, 
as  thereby  the  sharp  surfaces  presented  to  collect  the  fluid  will 
point  in  more  varied  directions. 

1038.  The  removal  of  silk  and  woollen  garments,  worn  during  the 
day  in  cold  weather,  is  often  accompanied  by  r.  slight  noise,  resem- 
bling that  of  sparks  issuing  from  a-  fire.     A  similar  effect  is  pro- 
duced on  passing  the  hand  softly  over  the  V,ck  of  a  cat.     These 
effects  are  produced  by  electricity. 

1039.  It  may  here  be  remarked,  that  the  verms  positive  and  nega- 
tive, are  merely  relative  terms,  as  applied  to  the  subject  of  electric- 
ity.    Thus,  a  'body   which   is  possessed   of  its  natural  share   of 
electricity,  is  positive  in  respect  to  one  that  has  less,  and   negative 
in  respect  to  one  that  has  more  than  its  natural  share  of  the  fluid. 
3o,  also,  one  that   has  more  than  its  natural  share  is  positive  with 
regard  to  one  that  has  only  its  natural  share,  or  less  *lian  its  natu- 
ral  share,  ,and  negative  in  respect  to  one   having  a  larger  share 
than  itself. 

1040.  The  experiments  with  the  spiral  tube  connected  with  Fig 
150  may  be  beautifully  varied  by  having  a  collection  of  such  tubes 
placed  on  a  stand  ;  and  ajar  coated  with  small  strips,  resembling  a 
brick  wall,  presents,  when  it  is  charged,  a  beautiful  appearance  ir, 
she  dark. 

1041.  The  electric  fluid  occupies  no  perceptible  space  of  time 
in  its  passage  through  its  circuit.     The  rapidity  of  its  motion  ha* 
been  estimated  as  high  as  288,000  miles  in  a  second  of  time.     B 
always  seem?  to  prefer  the  shortest  passage,  when  the  conductors 


ELECTKIC1TY.  2bl 

ire  equally  good.  Thus,  if  two,  ten,  a  hundred,  or  a  thousand  or 
more  persons,  join  hands,  and  be  made  part  of  the  chcuit  of  the  fluid 
in  passing  from  the  inside  to  the  outside  of  a  Leyden  jar,  they  will 
ill  feel  the  shock  at  the  same  moment  of  time.  But,  in  its  passage, 
the  Quid  always  prefers  the  best  conductors.  Thus,  if  two  clouds, 
iifterently  electrified,  approach  one  another,  the  fluid,  in  its  passage 
iTom  one  cloud  to  the  other,  will  sometimes  take  the  earth  in  its 
course,  because  the  air  is  a  bad  conductor. 

1042.  In  thunder-storms  the  electric  fluid  sometimes  passes  from 
the  clouds  to  the  earth,  and  sometimes  from  the  earth  to  the  clouds 
and  sometimes,  as  has  just  been  stated,  from  one  chnid  to  the  earth, 
and  from  the  earth  to  another  cloud.* 

What  are  1043.  It  is  not  safe,  during  a  thunder-storm,  to 

comparatively  take  shelter  under  a  tree,  because  the  tree  attracts 
safe  and  un-  the  fluid  and  the  human  body  being  a  better  con. 
safe  positions  .  * 

during  a  ductor  than  the  tree,  the  fluid  will  leave  the  tree 

thunder-storm?  and  pass  into  the  body. 

It  is  also  unsafe  to  hold  in  the  hand  edge-tools,  or  any  sharp 
point  which  will  attract  the  fluid. 

The  safest  position  that  can  be  chosen  during  a  thunder-storm 
is  a  recumbent  posture  on  a  feather  bed ;  and  in  all  situations  a 
recumbent  is  safer  than  an  erect  position.  No  danger  is  to  be 
apprehended  from  lightning  when  the  interval  between  the  flash 
and  the  noise  of  the  explosion  is  as  much  as  three  or  four  sec- 
onds. This  space  of  time  may  be  conveniently  measured  by  the 
beatings  of  the  pulse,  if  no  time-piece  be  at  hand. 

1044.  Lightning-rods  were  first  proposed  by  Dr.  Franklin,  to  whoa, 
is  also  ascribed  the  honor  of  the  discovery  that  thunder  and  light- 
ning are  the  effects  of  electricity.  He  raised  a  kite,  constructed  of  a 
silk  handkerchief  adjusted  to  two  light  strips  of  cedar,  with  a 
pointed  wire  fixed  to  it ;  and,  fastening  the  end  of  the  twine  to  a  key, 
and  the  key,  by  means  of  a  piece  of  silk  lace,  to  a  post  (the  silk  lace 
serving  to  insulate  the  whole  apparatus),  on  the  approach  of  a 

*  Lightning  appears  under  several  different  forms.  That  which  appears 
when  the  discharge  takes  place  between  two  clouds  at  some  distance  apart, 
or  between  a  cloud  and  the  earth,  exhibits  a  bright  zigzag  narrow  band  of 
light,  and  is  called  popularly  o/iam-lightning.  The  irregular  path  is  prob- 
ably caused  by  the  resistance  offered  Toy  the  air  to  the  current  or  electrical 
impulse.  When  two  clouds  slightly  charged  with  different  electrieitlos  ap- 
proach each  other,  the  discharge  is  from  a  great  many  points  at  the  same 
time,  and  the  appearance  of  a  broad  flash  has  entitled  it  to  the  name  of 
s7^-lightning.  A  third  form  of  lightning  is  called  JaZMightning  ;  no  satis- 
factory explanation  of  it  has  yet  been  given.  It  appears  like  a  bright  ball  of 
flame,  moving  no  faster  than  a  man  can  walk  ;  it  explodes  violently  after  a 
few  seconds. 


282  NATURAL    PHILOSOPHY. 

thunder- cloud,  he  was  able  to  collect  sparks  from  the  key,  to  charge 
Leyden  jars,  and  to  set  fire  to  spirits.  This  experiment  established 
the  identity  of  lightning  and  electricity.  The  experiment  was  a 
dangerous  one,  as  was  proved  in  the  case  of  Professor  Richman,  of 
St.  Petersburgh,  who  fell  a  sacrifice  to  his  zeal  for  electrical  science 
by  a  stroke  of  lightning  from  his  apparatus. 

What  are  the  1045.  Among  the  most  remarkable  facts  con- 
Electrical  nected  with  the  science  of  electricity,  may  be  men- 
Animals.  tioned  the  power  possessed  by  certain  species  of 
fishes  of  giving  shocks,  similar  to  those  produced  by  the  Leyden 
jar.  There  are  three  animals  possessed  of  this  power,, namely, 
the  Torpedo,  the  Gymnotus  Electricus  (or  Surinam  Eel),  and 
the  Silurus  Electricus.  But,  although  it  has  been  ascertained 
that  the  Torpedo  is  capable  of  giving  shocks  to  the  animal  sys- 
tem, similar  to  those  of  the  Leyden  jar,  yet  he  has  never  been 
made  to  afford  a  spark,  nor  to  produce  the  least  effect  upon  the 
most  delicate  electrometer.  The  Gymnotus  gives  a  small  but 
perceptible  spark.  The  electrical  powers  of  the  Silurus  are  in- 
ferior to  those  of  the  Torpedo  or  the  Gymnotus,  but  still  sufficient 
to  ghe  a  distinct  shock  to  the  human  system.  This  power  seems 
to  have  been  bestowed  upon  these  animals  to  enable  them  to 
secure  their  prey,  and  to  resist  the  attacks  of  their  enemies. 
Small  fishes,  when  put  into  the  water  where  the  Gymnotus  is 
kept,  are  generally  killed  or  stunned  by  the  shock,  and  swallowed 
by  the  animal  when  he  is  hungry.  The  Gymnotus  seems  to  be 
possessed  of  a  new  kind  of  sense,  by  which  he  perceives  whether 
the  bodies  presented  to  him  are  conductors  or  not.  The  consid- 
eration of  the  electricity  developed  by  the  organs  of  these  ani- 
mals of  the  aquatic  order,  belongs  to  that  department  called 
Animal  Electricity. 

1046.  It  will  be  recollected  that  the  phenomena  which  have 
now  been  described  with  the  exception  of  what  has  just  beer 
stated  as  belonging  to  animal  electricity,  belong  to  the  subject 
of  frictional  electricity.  But  there  are  other  forms  in  which 
this  subtle  agent  presents  itself,  which  are  yet  to  be  described, 
which  show  that  its  operations  are  not  confined  to  beautiful 


GALVANISM. 

experiments,  such  as  have  already  been  presented,  nor  to  the 
terrific  and  tremendous  effects  that  we  witness  in  tho  storm  and 
the  thunder-gust.  Its  powerful  agency  works  unseen  on  ihe 
intimate  relations  of  the  parts  and  properties  of  bodies  of  every 
description,  effecting  changes  in  their  constitution  and  character 
so  wonderfully  minute,  thorough  and  universal,  that  it  may 
almost  be  considered  as  the  chief  agent  of  nature,  the  prime 
minister  of  Omnipotence,  the  vicegerent  of  creative  power. 

What  is  1047.  GALVANISM,  OR  VOLTAIC  ELECTRIC- 

Gilvanism  ?  ITY.  —  Galvanism,  or  Voltaic  Electricity,  is  a 
branch  of  electricity  which  derives  its  name  from  Galvani, 
who  first  discovered  the  principles  which  form  its  basis. 

1048.  Dr.  Aloysius  Galvani  was  a  Professor  of  Anatomy  in  Bolog- 
na, and  made  his  discoveries  about  the  year  1790.  His  wife,  being 
consumptive,  was  advised  to  take,  as  a  nutritive  article  of  diet,  some 
soup  made  of  the  flesh  of  frogs.  Several  of  these  animals,  recently 
•skinned  for  that  purpose,  were  lying  on  a  table  in  his  laboratory, 
near  an  electrical  machine,  with  which  a  pupil  of  the  professor  was 
amusing  himself  in  trying  experiments.  While  the  machine  was  in 
action,  he  chanced  to  touch  the  bare  nerve  of  the  leg  of  one  of  the 
frogs  with  the  blade  of  a  knife  that  he  held  in  his  hand,  when  sud- 
denly the  whole  limb  was  thrown  into  violent  convulsions.  Galvani, 
being  informed  of  the  fact,  repeated  the  experiment,  and  examined 
minutely  all  the  circumstances  connected  with  it.  In  this  way  he 
was  led  to  the  discovery  of  the  principles'  which  form  the  basis  of 
this  science.  The  science  was  subsequently  extended  by  the  discov 
eiies  of  Professor  Volta,  of  Pavia,  who  first  constructed  the  galvanic 
or  voltaic  pile,  in  the  beginning  of  the  present  century. 

To  produce  electricity  mechanically  (as  has  been  stated  under  the 
head  of  frictional  electricity),  it  is  necessary  to  excite  an  electric  or 
non-conducting  substance  by  friction.  But  galvanic  action  is  pro- 
duced by  the  contact  of  different  conducting  substances  having  a 
chemical  action  on  one  another. 

How  does  gal-       1049.  Frictional  electricity  is  produced  by  the 
vanism  differ    mechanical  action  of  bodies  on  one  another ;  but 
%lk£icityl~  £alvanism»  or  galvanic  electricity,  is  produced  by 
their  chemical  action. 

What  is  the          1050.  The  motion  of  the  electric  fluid,  excited 

difference  in  by  galvanic    power     differs    from    tnat  explained 

(he  effects  of  J  _°     .  ... 

frictional  and  under  the  head  of  frictional   electricity  in  its  in- 

12* 


284  NATURAL   PHILOSOPHY. 

rJiemtcal  elec-  tensity  and  duration  ;  for,  while  the  xatter  exhibits 
tricity  ?  itself  in  sudden  and  intermitted  shocks  and  explo- 

sions, the  former  continues  in  a  constant  and  uninterrupted  cui- 
rent  so  long  as  the  chemical  action  continues,  and  is  interrupted 
only  by  the  separation  of  the  substances  by  which  it  is  produced.* 

1051.  The  nerves  and  muscles  of  animals   are 
What  is  most 
sensitive  to       most  easily  affected  by  the  galvanic  fluid ;  and  the 

the  galvanic     voltaic  or  galvanic  battery  possesses  the  most  sur- 
prising powers  of  chemical  decomposition. 

How  is  the          1052    The  galenic  faid.  or  influence,  is  ex- 

galvanic  jluia 

excited  ?  cited  by  the  contact  of  pieces  of  different  metal, 
and  sometimes  by  different  pieces  of  the  same  metal. 

1053.  If  a  living  frog,  or  a  fish,  having  a  slip  of  tin-foil  on  its  back, 
be  placed  upon  a  piece  of  zinc,  spasms  of  the  muscles  will  be  ex- 
cited whenever  a  communication  is  made  between  the  zinc  and  the 
tin-foil. 

1054.  If  a  person  place  a  piece  of  one  metal,  as  a  half-dollar, 
above  his  tongue,  and  a  piece  of  some  other  metal  (as  zinc)  below 
the  tongue,  he  will  perceive  a  peculiar  taste  ;  and,  in  the  dark,  will 

*  The  different  action  of  gravity  on  the  particles  of  water  while  in  the 
liquid  state,  and  the  same  particles  in  the  solid  state  in  the  form  of  ice,  has 
been  explained  in  the  early  pages  of  this  volume.  In  the  one  case  each 
particle  gravitates  independently,  while  in  the  form  of  ice  they  gravitate 
in  one  mass.  The  fall  of  a  body  of  ice  would  therefore  produce  more  serious 
injury  than  the  fall  of  the  same  quantity  of  water  in  the  liquid  form.  There 
is  a  kind  of  analogy  (which,  though  not  sufficient  for  a  philosophical  expla- 
nation, may  serve  to  give  an  insight  into  the  difference  between  the  effects 
produced  by  frictional  electricity  and  that  obtained  by  chemical  means.) 
between  the  gravitation  of  water  and  ice,  respectively,  and  the  motion  of 
frictional  and  chemical  electricity.  If  the  water  be  dropped  in  an  infinitely 
narrow  stream,  its  effects,  although  mechanically  equal,  would  be  so  gradual 
as  to  be  imperceptible.  So,  also,  if  a  given  portion  of  electricity  be  set  in 
motion  as  it  were  in  one  mass,  and  an  equal  quantity  move  in  an  infinitely 
narrow  current,  there  will  be  a  corresponding  difference  in  its  apparent 
results.  The  difference  in  intensity  may  perhaps  be  partially  understood  by 
this  illustration,  although  a  strict  analogy  may  fail  to  have  been  made  out, 
owing  in  part  to  the  nature  of  an  imponderable  agent.  A  strict  analogy 
cannot  exist  between  the  operations  of  two  agents,  one  of  which  is  pondera- 
ble and  the  other  imponderable.  But,  that  there  is  something  like  ar 
analogy  existing  in  the  cases  cited,  will  appear  from  statements  which  have 
been  made  on  good  authority,  namely,  that  there  is  a  greater  quantity  of 
electricity  developed  by  the  action  of  a  single  drop  of  acid  on  a  very  niiuuto 
portion  of  zinc,  than  ib  usually  brought  'f  to  action  in  the  darkest  cloud  thai, 
shroud?  the  hot  j SOD. 


GALVANISM. 

seo  a  flash  of  light  whenever  the  outer  edges  of  the  metals  are  in 
contact. 

1055.  A  faint  flash  may  be  made  to  appear  before  the  eyes  by 
putting  a  slip  of  tin-foil  upon  the  bulb  of  one  of  the  eyes,  a  piece  ot 
silver  in  the  mouth,  and  making  a  communication  between  then? 
In  these  experiments  no  effect  is  produced  so  long  as  the  metals  are 
kept  apart  ;  but,  on  bringing  them  into  contact,  the  effects  above 
described  are  produced. 

T;ir7  1056.  It  is  essential  in  all  cases  to  have  three 

What  is  es- 

sential lo  pro-  elements  to  produce  galvanic  action.  In  the  ex- 
duce  galvanic  periments  which  have  already  been  mentioned  in 
the  case  of  the  frogs,  the  fish,  the  mouth  and  the 
eye,  the  moisture  of  the  animal,  or  of  the  mouth,  supplies  the 
place  of  the  acid,  so  that  the  three  constituent  parts  of  the  circle 
are  completed. 


What  is  said  of     1057.  The  conductors  of  galvanic  electric- 

conductors  of  ity,  like  those  of  frictional  electricity,  are  of 
Galcanism?  -.,  1  m.  .  _  , 

all  degrees  ol  excellence.    The  metals,  char- 

coal, plumbago,  and  solutions  of  acids  and  salts,  are  good 
conductors  ;  while  gutta-percha,  rubber,  glass,  resin,  sul- 
phur, dry  wood,  air,  etc.,  are  poor  conductors,  or,  as  they 
are  generally  termed,  non-conductors.  Insulators  may  be 
made  of  any  of  the  above  solid  non-conductors. 

.      1058.  The  acid  employed  in  the  galvanic  cir- 
H  hat  kind  of  . 

acid  must  be     cult  must  always  be  one  that  has  a  strong  am  nit  y 

employed  in  for  one  of  the  metals  in  tho  circuit.  When  zinc 
is  employed,  sulphuric  acid  may  form  one  of  tbe 
three  elements,  because  that  acid  has  a  strong  affinity  for  zino. 
What  is  a  law  1059'  ^  certain  quantity  of  electricity  is  always 
of  chemical  developed  whenever  chemical  action  takes  plac*. 
action?  Between  a  fluid  and  a  solid  body.  This  is  a  gen- 

eral law  of  chemical  action  ;  and,  indeed,  it  has  been  ascertained 
that  there  is  so  intimate  a  connection  between  electrical  and 
chemical  changes,  that  the  chemical  action  can  proceed  only  to 
a  certain  extent,  unless  the  electrical  equilibrium,  which  has 
disturbed,  be  again  restored.  Hence,  we  find  that  in  the 


280 


NATURAL    PHILOSOIHY. 


simple,  as  well  as  in  the  compound  galvanic  circle,  the  oxydatioa 

of  the  zinc  proceeds  with  activity  whenever  the  galvanic  circle 

is  completed ;  and  that  it  ceases,  or  at  least  takes  place  very 

slowly,  whenever  the  circuit  is  interrupted. 

What  is  neces-      1060.  To  produce  any  galvanic  action  it  it 

ITexdte'gaL     necessary  to  form  what  is  called  a  galvanic 

vanic  action  ?  circle  ,  that  is,  a  certain  order  or  succession  of 

substances  capable  tf  exciting  electricity. 

Of  what  15  the      1061.   The  simplest  galvanic  circle  is  com- 

simplest  gal-     posed  of  three  conductors,  one  of  which  must 

be  solid5  and  °ne  fluid ; tne  third  may  be  either 

solid  or  fluid. 

What  is  the  1062.  The  process  usually  adopted  for  obtam- 
"fo^ohaininir  *n&  galvam'c  electricity  is,  to  place  between  two 
galvanic  eke-  plates  of  different  kinds  of  metal  a  fluid  capable 
tricity  ?  Of  exerting  some  chemical  action  on  one  of  the 

plates,  while  it  has  no  action,  or  a  different  action,  on  the  other 
A  communication  is  then  formed  between  the  two  plates. 
Explain  1063.  Fig.  156  represents  a 
*&'  '  simple  galvanic  circle.  It  con- 
sists of  a  vessel  containing  a  portion  of 
diluted  sulphuric  acid,  with  a  plate  of  zinc, 
Z,  and  of  copper,  C,  immersed  in  it.  The 
plates  are  separated  at  the  bottom,  and  the 
circle  is  completed  by  connecting  the  two 
plates  on  the  outside  of  the  vessel  by  means 
of  wires.  The  same  effect  will  be  pro- 
duced, if,  instead  of  using  the  wires,  the 
metallic  plates  come  into  direct  contact. 

1064.    In    the    above   ar- 
What  are  the 
essential  parts  rangement,    there    are   three 

of  a  galvanic  elements   or    essential    parts, 
namely,  the  zinc,  the  copper, 
end  the  acid      The   acid,  acting  chemical1,)  upon  the  zinc,  pro- 


Fig.  156. 


GALVANISM.  V&l 

duces  an  alteration  in  the  electrical  state  of  the  metal.  The 
zinc,  communicating  its  natural  share  of  zhc  electrical  fluid  to 
the  acid,  becomes  negatively  electrified.  The  copper,  attracting 
the  same  fluid  from  the  acid,  becomes  positively  electrified.  Any 
conducting  substance,  therefore,  placed  within  the  line  of  com- 
munication between  the  positive  and  negative  points,  will  re- 
ceive the  charge  thus  to  be  obtained.  The  arrows  in  Fig.  156 
show  the  direction  of  the  current  of  positive  electricity,  namely, 
from  the  zinc  to  the  fluid,  from  the  fluid  to  the  copper,  from 
the  copper  back  through  the  wires  to  the  zinc,  passing  from 
zinc  to  copper  in.  the  acid,  and  from  copper  to  zinc  out  of  the 
acid.  The  substance  submitted  to  the  action  of  the  electric  cur- 
Where  must  a  rent  must  be  placed  in  the  line  of  communication 
substance  be  between  the  copper  and  the  zinc.  The  wire  con- 
affected  by  *al- nected  w^tn  tae  copper  is  called  the  positive  poler 
vanic  action  ?  and  that  connected  with  the  zinc  the  negative  pole, 
and  in  all  cases  the  substance  submitted  to  galvanic  action  must 
be  placed  between  the  positive  and  negative  poles. 

1065.  The  electrical  effects  of  a  simple  galvanic  circle,  such  as 
has  now  been  described,  are,  in  general,  too  feeble  to  be  perceived, 
except  by  very  delicate  tests.  The  muscles  of  animals,  especially 
those  of  cold-blooded  animals,  such  as  frogs,  &c.,  the  tongue,  the 
eye,  and  other  sensitive  parts  of  the  body,  being  very  easily 
affected,  afford  examples  of  the  operation  of  simple  galvanic 
circles.  In  these,  although  the  quantity  of  electricity  set  in 
motion  is  exceedingly  small,  it  is  yet  sufficient  to  produce  verj 
considerable  effects ;  but  it  produces  little  or  no  effect  on  the 
most  delicate  electrometer. 

1066.  The  galvanic  effects  of  a  simple  circle 

H^w  may  gat-  °  r        ,  . 

vanic  action  be  may  be  increased  to  any  degree,  by  a  .repetition 
increased?  of  tjie  game  simpie  combination.  Such  repe- 
titions constitute  compound  galvanic  circles,  and  are  called 
galvanic  piles,  or  galvanic  batteries,  according  to  the  mode 
in  which  they  are  constructed. 


288  NATURAL    PHILOSOPHY. 

1.0G7.  It  appears  a  \  first  view  to  be  a  singular  fact,  that,  in  a  simple 
galvanic  circle,  composed  of  zinc,  acid  and  copper,  the  zinc  enr» 
will  always  be  negative,  anil  the  copper  end  positive ;  while,  in  all 
compound  galvanic  circles  composed  of  the  same  elements,  the  zinc 
will  be  positive,  and  the  copper  negative.  This  apparent  difference 
arises  from  the  compound  circle  being  usually  terminated  by  two 
superfluous  plates. 

What  is  the  1068.  The  voltaic  pile  consists  of  alternate 
Voltaic  pile  ?  places  of  two  different  kinds  of  metal,  sepa- 
rated by  woollen  cloth,  card,  or  some  similar  substance. 

Explain         1069.  Fig.  157  represents  a  voltaic  Fig.  157. 

Fig.  157.  pile.  A  voltaic  pile  may  be  con- 
structed in  the  following  manner  :  Take  a 
number  of  plates  of  silver,  and  the  same  num- 
ber of  zinc,  and  also  of  woollen  cloth, —  the  oloth 
having  been  soaked  in  a  solution  of  sal  ammo- 
niac in  water.  With  these  a  pile  is  to  be  formed,  in  the  following 
order,  namely  :  a  piece  of  silver,  a  piece  of  zinc,  a  piece  of  cloth, 
and  thus  repeated.  These  are  to  be  supported  by  three  glass 
rods,  placed  perpendicularly,  with  pieces  of  wood  at  the  top  and 
bottom,  and  the  pile  will  then  be  complete,  and  will  afford  a 
constant  current  of  electric  fluid  through  any  conducting  sub- 
stance. Thus,  if  one  hand  be  applied  to  the  lower  plate,  and 
the  other  to  the  upper  one,  a  shock  will  be  felt,  which  will  be 
repeated  as  often  as  the  contact  is  renewed. 

Instead  of  silver,  copper  plates,  or  plates  of  other  metal,  may 
be  used  in  the  above  arrangement.  The  arrows  in  the  figure 
show  the  course  of  the  current  of  electricity  in  the  arrangement 
of  silver,  zinc,  &c. 

1070.  Voltaic  piles  have  been  constructed  of  layers  of  gold 
and  silver  paper.  The  effect  of  such  piles  remains  undisturbed 
for  years.  With  the  assistance  of  two  such  piles,  an  approxi- 
mation to  perpetual  motion,  in  a  self-moving  clock,  has  been  in- 
vented by  an  Italian  philosopher.  The  motion  is  produced  by 
the  attraction  and  repulsion  of  the  piles  exerted  on  a  pith-ball, 
on  the  principle  of  the  electrical  bfclls.  The  ton  of  one  of  tht« 


GALVANISM. 

piles  was  positive,  and  the  bottom  negative.  T\e  other  pile  was 
in  an  opposite  state ;  namely,  the  top  negative,  and  the  bottom 
positive. 

W\aih  the  1071.  The  voltaic,  or  galvanic  battery,  is  a 
galvanic  bat-  combination  of  metallic  plates,  immersed  in 
tcry  •  pairs  in  a  fluid  which  exerts  a  chemical  action 

on  one  of  each  pair  of  the  plates,  and  no  action,  or,  at  least, 
a  different  action,  on  the  other. 

What  is  the  1072.  The  electricity  excited  by  the  battery 
diction  of  the  i  j  th  Ud  t  ^  fl  id  M  h  t 

current  in  the    r  * 

galvanic  bat-    upon  it  chemically.     Thus,  in  a  battery  composed 

tery  *  of  zinc,  diluted  sulphuric  acid  and  copper,  the  acid 

lets  upon  the  zinc,  and  not  on  the  copper.  The  galvanic  fluid 
proceeds,  therefore,  from  the  zinc  to  the  acid,  from  the  acid  to 
the  copper,  &c.  Instead  of  using  two  different  metals  to  form 
the  galvanic  circuit,  one  metal,  in  different  states,  may  be  em- 
ployed ;  —  the  essential  principle  being,  that  one  of  the  elements 
shall  be  more  powerfully  affected  by  some  chemical  agent  than 
the  other.  Thus,  if  a  galvanic  pair  be  made  of  the  same  metal, 
one  part  must  be  softer  than  the  other  (as  is  the  case  with  cast 
and  rolled  zinc) ;  or  a  greater  amount  of  surface  must  be  exposed 
to  corrosion  on  one  side  than  on  the  other ;  or  a  more  powerful 
chemical  agent  be  used  on  one  side,  so  that  a  current  will  be 
sent  from  the  part  most  corroded,  through  the  liquid,  to  the  part 
least  corroded,  whenever  the  poles  are  united,  and  the  circuit 
thereby  completed.  " 

Explain  1073.  Fig.  158  represents  Fig.  158. 

Fig.  158.  a  voltaic  battery.  It  con- 
sists of  a  trough  made  of  baked  wood, 
wedgewood-ware,  or  some  other  non- 
conducting substance.  It  is  divided 
into  grooves,  or  partitions,  for  the  re- 
ception of  the  acid,  or  a  saline  solution) 
and  the  plates  of  zinc  or  copper  (or 
other  metal)  are  iniiner«ed  by  pairs  in  the  grooves.  Ti'cse 


290  NATURAL    PHILOSOPHY. 

pairs  of  plates  are  united  by  a  slip  of  metal  passing  from  the 
one  and  soldered  to  the  other  ;  each  pair  being  placed  so  aj  to 
enclose  a  partition  between  them,  and  each  cell  or  groove  in  tho 
trough  containing  a  plate  of  zinc,  connected  with  the  copper 
plate  or  the  succeeding  cell,  and  a  copper  plate  joined  with  the 
ainc  plate  of  the  preceding  cell.  These  pairs  must  commence 
with  copper  and  terminate  with  zinc,  or  commence  with  zinc  and 
terminate  with  copper.  The  communication  between  the  firsl 
and  last  plates  is  made  by  wires,  which  thus  complete  the  gal- 
vanic circuit.  The  substance  to  be  submitted  to  galvanic  action 
is  placed  between  the  points  of  the  two  wires. 

How  can  a  1074.  A  compound  battery  of  great  power  is 

compound  bat-  obtained  by  uniting  a  number  of  these  troughs. 

tery  of  great     _  .    ./ 

power  be  ob-      ^  a  similar  manner,  a  battery  may  be  produced 

tained?  by  uniting  several  piles,  making  a  metallic  com- 

munication between  the  last  plate  of  the  one  and  the  first  plate 
of  the  next,  and  so  on,  taking  care  that  the  order  of  succession 
of  the  plates  in  the  circuit  be  preserved  inviolate. 

Describe  the  1075-  The    ^ouronm  Kg.  159> 

Couronne  des    des  tosses,  represented  in 
iasses.  Fig  159)  ig  anotacr  form 

of  the  galvanic  battery.  It  consists  of 
a  number  of  cups,  bowls,  or  glasses, 
with  the  zinc  and  copper  plates  im- 
mersed in  them,  in  the  order  represent- 
ed in  the  figure ;  Z  indicating  the  zinc, 
and  C  the  copper  plates ;  the  arrows  denoting  the  course1,  of  the 
electric  fluid. 

1076.  The  electric  shock  from  the  voltaic  battery  may  bo 
received  by  any  number  of  persons,  by  joining  hands,  having 
previously  wetted  them. 

Describe  Smee's  1077.  SMEE'S  GALVANIC  B ATI ERY  13  represented 
Battery,  in  Fig.  160,  and  affords  an  instance  of  a  battery 

in  its  simplest  form.  It  consists  of  a  glass  vessel  (AS  a  tumbler), 
i>n  \viiich  rest*  the  frame  that  supports  the  apparatus 


GALVANISM. 


Two  screw-v3ups  rise  from  the  frame,  to  which  Fig.  ieo. 

wires  may  be  attached  for  the  conveyance  of 
the  electric  current  in  any  direct  ice.  One  of 
the  screw-cups  communicates  with  a  thin  strip 
of  platinum,  or  platinum-foil,  which  is  sus- 
pended within  the  glass  vessel  between  two 
plates  of  zinc,  thus  presenting  each  surface  of 
the  platinum  to  a  surface  of  zinc  ;  and  the  gal- 
vanic action  is  in  proportion  to  the  extent  of  the  opposite  sur 
faces  of  the  two  metals,  and  their  nearness  to  each  other.  The 
other  screw-cup  is  connected  with  the  two  zinc  plates.  The 
screw-cup  connected  with  the  platinum  is  insulated  from  the 
metallic  frame  which  supports  it,  by  rosewood,  and  a  thumb- 
screw confines  the  zinc  plates,  so  that  they  can  be  renewed  when 
necessary.  The  liquid  employed  for  this  battery  is  sulphuric 
acid,  or  oil  of  vitriol,  diluted  with  ten  parts  of  water  by  measure. 
To  prevent  the  action  of  the  acid  upon  the  zinc  plates,  their  sur- 
faces are  commonly  amalgamated,  or  combined  with  mercury 
which  prevents  any  chemical  action  of  the  acid  with  the  zino 
until  the  galvanic  circuit  is  established,  when  the  zino  is  imme- 
diately attacked  by  the  acid. 

Explain  1078.  Fig.  161  represents  a  series  of  three  pairs 

Fig.  161.     of  this  battery,  in  which  it  will  be  observed  that  the 

Fig.  161. 


platinum  of  one  is  connected  with  the  zinc  of  the  next,  and  that 
the  terminal  wires  proceed,  consequently,  one  from  a  platinum 
jjhte,  and  the  other  from  a  zinc  plate,  as  iii  a  single  pair. 


2D2 


NATURAL    PHILOSOPHY. 


Describe  tht 
sulphate  of 
topper   bat- 
tery by 
Figures  162 
and  163 


1078.  SULPHATE  OF   COPPER   BATTERY.  —  Fig 
162  represents  a  sulphate  of  copper  battery,  and 
Fig.  163  a  vertical  section  of  the  same  battery. 
It  consists  of  a  double  cylinder  of  copper,  C  C, 
Fig.  163,  with  a  bottom  of  the  same  metal,  which 

Fig.  162. 


Fig.  163. 


serves  the  double  purpose  of  a  gal- 
panic  plate  and  a  vessel  to  contain 
the   exciting  solution.     The  solu- 
tion is  contained  in  the  space  be- 
tween the  two  copper  cylinders.     A 
movable  cylinder  of  zinc,  Z,  is  let 
down  into  the   solution  whenever 
the  battery  is  to  be  used.     It  rests 
on  three  arms  of  wood  or  ivory  at 
the  top,  by  means  of  which  it  is  in- 
sulated.    Thus   suspended  in   the 
solution,  the  surfaces  of  zinc  and 
copper,  respectively,  face  each 
other.     A  screw-cup,  N,  is  at- 
tached to  the  zinc,  and  anoth- 
er, P,  to  the  copper  cylinder, 
to  receive  the  wires.     When 
a  communication  is  made  be- 
tween the  two  cups,  electricity 
is   excited.     The  liquid   em- 
ployed   in   this  battery  is    a 
solution  of  sulphate  of  copper 
(common     blue     vitriol)     in  water.      A  saturated   solution    is 
first  made,  and  to  this  solution  as  much  more  water  is  added. 

1079.  A  pint  of  water  will  dissolve  about  a  quarter  of  a  pound  oi 
blue  vitriol.  The  solution  described  above  will  therefore  contain 
about  two  ounces  of  the  salt  to  the  pint.  The  addition  of  alcohol 
in  small  quantities  increases  the  permanency  of  the  action  of  the 
oolution  The  zinc  cylinder  bhould  always  betaken  out  of  the  solu- 
tion when  the  battery  is  not  in  use  ;  but  the  solution  may  remain 
in  the  battery  The  battery  will  keep  in  good  action  ft  r  tw?uty  or 
thirty  minutes  at  a  time 


GALVANISM.  29.S 

1080.  The  sulphate  of  copper  battery,  although  not  so  ener- 
getic as  Smee's,  is  found  very  convenient  ir.  a  large  class  of 
experiments,  and  is  particularly  recommended  to  those  who  are 
inexpert  in  the  use  of  acids ;  because  the  sulphate  of  copper,  being 
entirely  neutral,  will  not  injure  the  color  nor  the  texture  of 
organic  substances. 

Describe  the  1081.  There  is  another  form  of  the  sulphate  of 
protected  sul-  copper  battery,  called  the  Protected  Sulphate  of 
phate  of  cop-  Copper  Battery,  which  differs  from  the  one  described 
•'  in  having  a  porous  cell  01  earthenware,  or  leather, 
interposed  between  the  zinc  and  the  copper,  thus  forming  two 
cells,  in  the  outer  of  which  sulphate  of  copper  may  be  used,  and 
in  the  inner  one  a  solution  of  sulphate  of  soda  (Glauber  salt), 
or  chloride  of  sodium  (common  salt),  or  even  dilute  sulphuric 
acid.  This  battery  will  continue  in  use  for  several  days,  and  it 
is  therefore  of  great  use  in  the  electrotype  process. 

1082.  GROVE'S  BATTERY. —  This  is  the  most 
*   energetic  battery  yet  known,  and  is  the  one 
most  generally  used  for  the  magnetic  telegraph. 
The  metals  employed  are  platinum  and  zinc,  and  the  solutions 
are  strong  nitric  acid  in  contact  with  the  pla- 
tinum, and  sulphuric  acid  diluted  with  ten  or  Fig> 
twelve  parts  of  water  in  contact  with  the  zinc. 
This  battery  must  be  used  with  great  care,  on 
account  of  the  strength  of  the  acids  used  for 
the  solutions,  which  send  out  injurious  fumes, 
and  which   are   destructive   to  organic  sub- 
stances.    Fig.    164  represents   Grove's   bat- 
tery.    The  containing  vessel  is  glass ;  within 
this  is  a  thick  cylinder  of  amalgamated  zinc,  standing  on  short 
legs,  and  divided  by  a  longitudinal  opening  on  one  side,  in  order 
to  allow  the  acid  to  circulate  freely.     Inside  of  this  is  a  porous 
cell  of  unglazed  porcelain,  containing  the  nitric  acid,  and  strip 
uf  platinum.     The  platinum  is  supported  by  a  strip  of  bras? 
fixed  by  a  thumb-screw  and  an  insulating  piece  of  ivory  to  the 


294  NATURAL    PHILOSOPHY. 

arm  proceeding  from  the  zinc  cylinder.  The  amalgamated  z:nc 
if  not  acted  upon  by  the  diluted  sulphuric  acid  until  the  circuit 
of  the  battery  is  completed.  But,  as  the  nitric  acid  will  filter 
through  the  porous  cell,  and  act  upon  the  zinc,  it  is  advisable  to 
remove  the  zinc  from  the  acid  when  the  battery  is  to  remain 
inactive.  The  action  of  Grove's  battery  may  be  considered  as 
three  times  greater  than  that  of  the  sulphate  of  copper  battery. 
What  are  the  1083.  The  spark  from  a  powerful  voltaic  bat- 
effecls  of  a  pow-  tery  acts  upon  and  inflames  gunpowder,  char- 

erful  voltai-  bat-    coalj  cotton ,  and  other  inflammable  bodies,  fuses 

tery?  .     ;  ..  ..  ,         , 

all  metals,  ourns  up  or  disperses  diamonds  and 

other  substances  on  which  heat  in  other  forms  produces  little  or 
no  effect. 

1084.  The  moat  striking  effects  of  Galvanism  on  the  human 
frame,  aftor  death,  were  exhibited  at  Glasgow,  a  few  years  ago. 
The  subject  on  which  the  experiments  were  made  was  the  body  of 
the  murderer  Clydesdale,  who  was  hanged  at  that  city.  He  had 
Deen  suspended  an  hour,  and  the  first  experiment  was  made  in 
about  ten  minutes  after  he  was  cut  down.  The  galvanic  battery 
employed  consisted  of  270  pairs  of  four-inch  plates.  On  the  appli- 
cation of  the  battery  to  different  parts  of  the  body,  every  muscle 
was  thrown  into  violent  agitation  ;  the  leg  was  thrown  out  with 
great  violence,  breathing  commenced,  the  face  exhibited  extraordi- 
nary grimaces,  and  the  finger  seemed  to  point  out  the  spectators. 
Many  persons  were  obliged  to  leave  the  room  from  terror  or  sick- 
ness ;  one  gentleman  fainted,  and  some  thought  that  the  body  had 
really  come  to  life. 

1085.   The  wires,  by  which  the  circuit  of  the 
How  are  the  '    J 

hands  protected   battery  is  completed,  are  generally  covered  with 

when  using  a  gutta-percha,  in  order  that  they  may  be  held  or 
lattery  ? 

directed  to  any  substance. 

(n  what  respects  1086'  There  are  three  FinciPal  circum- 
loes  the  electric-  stances  in  which  the  electricity  produced  by 

ity  produced  by  the  gaivanic  or  voltaic  battery  differs  from 
the  galvanic  bat-  ,  , 

iery  differ  from    tnat  obtained  by  the  ordinary  electrical  ma- 

ihat  obtained  by    chine ;  namely, 

the  machine  ?  ^  The  yery  bw  degree  of  intenslfy  of  tnat 

produced  by  the  galvanic  battery,  compared  with  that  obtained 
h\  the  machine 


GALVANISM. 

1087  By  inte7isity  is  here  meant  something  analogous  to 
what  is  implied  by  density  as  applied  to  matter ;  but  in  the  ono 
case  it  is  a  ponderable  agent,  in  the  other  an  imponderable,  so 
that  a  strict  analogy  cannot  be  made  out  between  them.  The 
term  density  cannot  be  applied  to  any  of  the  imponderable 
agents,  light,  sound,  heat  or  electricity.  We  speak  of  the  in- 
tensity of  light,  an  intensity  of  heat,  &c.  Hence,  the  word 
intensity  is  properly  applied  to  electricity,  and  we  speak  of  its 
tension,  instead  of  its  density. 

•Which  will  de-        The  quantity  Of  electricity  obtained  by  gal- 
velop  the  great-  ....••.  , 

er  quantity   of     vamc    action    is   much    greater    than  can  be 

electricity,   the      obtained  by   the  machine;  but  it  flows,  as  it 
f±t±3    were,  in  narrow  stream,    ' 

The  action  of  the  electrical  machine  may  be  compared  to  a  mighty 
torrent,  dashing  and  exhausting  itself  in  one  leap  from  a  precipitous 
height.  The  galvanic  action  may  be  compared  to  a  steady  stream, 
supplied  by  an  inexhaustible  fountain.  In  other  words,  the  mo- 
mentum of  the  electricity  excited  by  galvanism  is  less  than  that 
from  the  electrical  machine  ;  but  the  quantity,  as  has  been  stated. 
is  greater. 

(2.)  The  very  large  quantity  of  electricity  which  is  set  in  mo- 
tion by  the  voltaic  battery ;  and, 

(3.)  The  continuity  of  the  current  of  voltaic  electricity,  and 
its  perpetual  reproduction,  even  while  this  current  is  tending  to 
restore  the  equilibrium. 

1088.  Whenever  an  electrical  battery  is  charged,  how  great 
soever  may  be  the  quantity  that  it  contains,  the  whole  of  the 
power  is  at  once  expended,  as  soon  as  the  circuit  is  completed. 
Its  action  may  be  sufficiently  energetic  while  -it  lasts,  but  it  is 
exerted  only  for  an  instant,  and,  like  the  destructive  operation 
of  lightning,  can  effect  during  its  momentary  passage  only  sud- 
den and  violent  changes,  which  it  is  beyond  human  power  to 
regulate  or  control.  On  the  contrary,  the  voltaic  battery  con- 
tinues, for  an  indefinite  time,  to  develop  and  supply  vast  quan- 
tities of  electricity,  which,  far  from  being  lost  by  returning  to 
their  source,  circulate  in  a  perpefual  «tream  and  with  uudiiuiu- 


'296  NATUHAL    PHlLUSOPIiY 

ished  force.  The  effects  of  this  continued  current  on  the  bodie>s 
subjected  to  its  action  will  therefore  be  more  definite,  and  will 
be  constantly  accumulating ;  and  their  amount,  in  process  of 
time,  will  be  incomparably  greater  than  even  those  of  the  ordi- 
nary electrical  explosion.  It  is  therefore  found  that  changes  ii 
the  composition  of  bodies  are  effected  by  galvanism  which  car 
be  accomplished  by  no  other  means.  The  science  of  galvanism 
therefore,  has  extended  the  field  and  multiplied  the  means  ot 
investigation  in  the  kindred  sciences,  especially  that  of  Chem 
istry. 

1089.  A  common  electrical  battery  may  bo 
charged  from    a   voltaic   battery  of   sufficient 

sion  manifested  size  ;  but  a  battery  constructed  of  a  small  num- 
in  the  galvanic  ber  Of  pairs,  even  though  the  plates  are  large, 
furnishes  no  indication  of  attraction  or  repul- 
sion equal  to  that  which  is  given  by  the  feeblest  degree  of 
excitation  to  a  piece  of  sealing-wax.  A  galvanic  battery  con- 
sisting of  fifty  pairs  of  plates  will  affect  a  delicate  gold-leaf 
electrometer;  and,  with  a  series  of  one  thousand  pairs,  even 
pith  balls  are  made  to  diverge. 

1090.  The  effect  of  the  voltaic  pile  on  the 
On  what    does                  ,11  . 

the  effect  of  the    animal  body  depends  chiefly  on  the  number  of 

voltaic     battery    plates  that  are  employed;  but  the  intensity  of 
e?en  the  spark  and  its  chemical  agencies   increase 

more  with  the  size  of  the  plates  than  with  their  number. 

1091.  Galvanism    explains    many   facts    in 
Mention  some  of 

the  familiar  ef-     common  life. 

fects  ofgalvan-  •  Porter,  ale,  or  strong  beer,  is  said  to  have  a 
peculiar  taste  when  drunk  from  a  pewter  ves 
sel.  The  peculiarity  of  taste  is  caused  by  the  galvanic  circle 
formed  by  the  pewter,  the  beer,  &c.,  and  the  moisture  of  the 
under  lip. 

Works  of  metals  the  parts  of  which  are  soldered  together 
soon  tarnish  in  the  places  where  the  metals  are  joined. 

Ancient  coins  composed  of  a  mixture  of  metal  have  cruiu- 


GALYAJS'ISM. 

bled  to  pieces,  while  those  composed  )i  pure  metal  have  been 
uninjured. 

The  nails  and  the  copper  in  sheathing  of  ships  are  soon 
corroded  about  the  place  of  contact.  These  are  all  the  effects 
of  galvanism. 

There  are  persons  wno  profess  to  be  able  to  find  out  seams  in 
brass  and  copper  vessels  by  the  tongue  which  the  eye  cannot 
discover  ;  and,  by  the  same  means,  to  distinguish  the  base  mix- 
tures which  abound  in  gold  and  silver  trinkets. 

1092.  From  what  has  now  been  stated,  it  will  be  seen  that 
the  effects  of  galvanic  action  depend  on  two  nrcumstances ; 
namely,  1st,  the  size  of  the  plates  employed  ii  i  the  circuit ; 
and,  2dly,  the  number  of  the  pairs  constituting  a  battery.  But 
there  is  a  remarkable  circumstance  to  be  noticed  in  this  con- 
nexion ;  namely,  that  there  is  one  class  of  facts  dependent  on 
the  extension  of  the  size  of  the  plates,  and 
On  what  does  another  on  the  increase  of  their  number.  The 
the  power  of  a  f  deveiop  faat  and  7nagnetism  is  de- 

battery   to  pro-    e 
auce  heat  and  to    pendent  on  the  size  of  the  plates,  that  is,  on  the 

affect  the  animal    extent  of  the  surface  acted  upon  by  the  chem- 

system    respect-     .  ..  . 

ively  depend  ?        lca*    agen^  J    while    the    power    to    decompose 

chemical  compounds,  and  to  affect  the  animal 
system,  is  affected  in  a  greater  ratio  by  the  increase  of  the 
number  of  the  pairs. 

1093.  The  name  Color imotor   (that   is,  the 

heat^    WaS   aPPlied   bv   Dr'  Hare'  of 
Philadelphia,  to  a  very  powerful  apparatus  which 

he  constructed,  with  large  plates,  and  which  he  found  possessed 
of  a  very  remarkable  power  in  producing  heat.  Batteries  con- 
structed for  this  purpose  usually  consist  of  from  one  to  eight 
pairs  of  plates.  They  are  made  in  various  forms;  sometime? 
the  sheets  of  copper  and  zinc  are  coiled  in  concentric  spirals, 
sometimes  placed  side  by  side ;  and  they  may  be  divided  into  a 
great  number  of  small  plates,  provided  that  all  the  zinc  plates 
are  connected  together,  and  all  tlie  copper  plates  together,  and 


tfATUKAL   PHILOSOPHY. 

then  tho.t  the  experiments  are  performed  in  a  channel  oj  com* 
munication,  opened  between  the  SETS  OF  PLATES,  and  not  between 
PAIIIS,  as  in  the  common  battery  ;  for  it  is  immaterial  whether 
one  large  surface  be  used,  or  many  small  ones  electrically  con- 
nected together.  The  effect  of  all  these  arrangements,  by  which 
the  metallic  surface  of  a  single  pair  is  augmented,  is  to  increase 
the  quantity  produced. 

1094.  The  galvanic  or  voltaic  battery  is  one  of  the  most  valuable 
acquisitions  of  modern  science.  It  has  proved  in  many  instances 
the  key  by  which  science  has  entered  into  the  innermost  recesses  of 
nature,  and  discovered  the  secret  of  many  of  her  operations.  It 
has,  in  great  measure,  lifted  the  hitherto  impenetrable  veil  that  has 
concealed  the  mysterious  workings  in  the  material  world,  and  has 
opened  a  field  for  investigation  and  discovery  as  inviting  as  it  is 
boundless.  It  has  strengthened  the  sight  and  enlarged  the  view  of 
the  philosopher  and  the  man  of  science,  and  given  a  degree  of  cer- 
tainty to  scientific  inquiry  hitherto  known  to  be  unreached,  and  sup- 
posed to  be  unattainable  ;  and,  if  it  has  not  yet  satisfied  the  hopes 
of  the  alchemist,  nor  emulated  the  gold-converting  touch  of  Midas, 
it  has  shown,  almost  to  demonstration,  that  science  may  yet  achieve 
wonders  beyond  the  stories  of  mythology,  and  realize  the  familial 
adage  that  "  truth  is  stranger  than  fiction 

1095.  MAGNETISM.  —  Magnetism  treata 
netiim  "     aff~    °f  tnc  properties  and  effects  of  the  magnet, 
or  loadstone. 

1096.  The  term  loadstone,  or,  more  properly,  leadstone,  was  ap- 
plied to  an  ore  of  iron  in  the  lowest  state  of  oxidation,  from  its 
attractive  properties  towards  iron, -and  its  power  of  communicating 
its  power  to  other  masses  of  iron.     It  received  the  name  of  Magnet 
from  Magnesia,  in  Asia  Minor  (now  called  Guzelhizar) ,  about  fif- 
teen miles  from  Ephesus,  where  its  properties  were  first  well  known. 
The  term  magnet  is  now  applied  to  those  substances  which,  natu- 
rally or  artificially,  are  endowed  either  permanently  or  temporarilv 
with  the  same  attractive  power. 

1097.  Certain  ores  of  iron  are  found  to  be  naturally  pos- 
sessed oi  magnetic  properties,  and  are  therefore  called  natural 
or  native  magnets,  or  loadstones.     Besides  iron  and  some  of  the 
compounds    nickel,  and,  perhaps,  cobalt,  also  possess   magnetic 
properties.     But  al.  conductors  of  electricity  are  capable  of 
exerting  the  magnetic  properties  of  attraction   and   rcpultdon 


while  conveying  a  current  of  electricity,  as  will  be  shown  uncLr 
the  head  of  Electro-Magnetism. 

1098.  That  part  of  science  which  relates  to  the  development  of 
magnetism  by  means  of  a  current  of  electricity  will  be  noticed  ufi- 
der.the  head  of  Electro-Magnetism,  in  which  connexion  will  also 
be  mentioned  the  development  of  electricity  by  magnetism,  to  which 
the  term  Magneto-Electricity  has  been  applied. 

What  are   the        1099.  There  are  two  kinds  of  magnets, 
two   kinds   of    namely,  the  native  or  natural  magnet,  an<3 

the  artificial. 

1100.  The  native  magnet,  or  loadstone,  is  an  ore  of  iron, 
found  in  iron  mines,  and  has  the  property  of  attracting 
*ron,  and  other  substances  which  contain  it. 

What  is  a  per-       1101.  A  permanent  artificial  magnet  is  a 
manent  magnet?  piece  of  iron  to  which  permanent  magnetic 

properties  have  been  communicated. 

°f 


permanent  periment,  the  artificial  is  to  be  preferred  to 

or  the  artificial     ^          u       magnet 
magnet  ? 

1103.  If  a  straight  bar  of  soft  iron  be  held  in  a  vertical  posi- 
tion (or,  still  better,  in  a  position  slightly  inclined  to  the  perpen- 
dicular, the  lower  end  deviating  to  the  north),  and  struck  several 
smart  blows  with  a  hammer,  it  will  be  found  to  have  acquired, 
by  this  process,  all  the  properties  of  a  magnet;  or,  in  other 
words,  it  will  become  an  artificial  magnet. 

What    are    the         1104.  The    properties   of  a   magnet    are,  — 
properties  of  a    polarity  ;  attraction    of  unmagnetic   iron  ;    at- 

traction and  repulsion  of  magnetic  iron  ;  the 
power  of  communicating  magnetism  to  other  iron.  Beside* 
these  properties,  the  magnet  has  recently  been  discovered  to  be 
possessed  of  electrical  properties.  These  will  be  considered  it 
another  connexi  Dn. 

What  is  the  po-         1105.  By  the  polarity  of  a  magnet  is  meant 
larity  of  a  mag-     the    property    of   pointing    or    turning    to    the 

north    and  south  poles.     The  end  which  points 
13 


800  NATUKAJ 

to  the  north  is  called  the  north  pole  of  the  magnet,  and  the 
other  the  south  pole. 

1106.  The  attractive  powor  of  a  magnet  is  generally  stated 
to  be  greatest  at  the  poles  ;  but  the  actual  poles,  or  points  of 
greatest  magnetic  intensity,  in  a  steel  magnet,  are  not  exactly 
at  the  ends,  but  a  little  witnm  them. 

How  willa  mag-       1107-    When  a  magnet  is  supported  in 
net   move  when  such  a  manner  as  to  move  freely,  it   will 
eeysuspen         spontaneously   assume    a    position  directed 
nearly  north  and  south. 

1108.  The  points  to  which  the  poles  of  a 
What    are    the  .,  7        ml 
magnetic  poles  /    magnet  turn  are  the  magnetic  poles.  These 

do  not  exactly  coincide  with  the  astronomical 
poles  of  the  earth  ;  but,  although  the  value  of  the  magnetic 
needle  has  been  predicated  on  the  supposition  that  its  polar- 
ity is  a  tendency  to  point  exactly  to  the  north  and  south 
poles  of  the  earth,  the  recent  discovery  of  the  magnetic 
poles,  as  the  points  of  attraction,  has  not  depreciated  the 
value  of  the  compass,  because  the  variation  is  known,  and 
proper  allowances  can  be  made  for  such  variation. 

1109.  There  are  several  ways  of  supporting 

How  are  mag-  magriet,  so  as  to  enable  it  to  manifest  its 
nets  supported] 

polarity,     first,  b,y  suspending  it,   accurately 

balanced,  from  a  string.  Secondly,  by  poising  it  on  a  sharp 
point.  Thirdly,  by  attaching  it  to  some  buoyant  substance,  and 
allowing  it  to  float  freely  on  water. 

of  magnetic  "at-        1110.  Different  poles  of  magnets  attract, 
traction  and  re-    and  similar  poles  repel  each  other. 
pulsion  ? 

There  is  here  a  close  analogy  between  the  attractive  and  repul- 
sive powers  of  the  positive  and  the  negative  forms  of  electricity, 
and  the  northern  and  southern  polarities  of  the  magnet.  The  same 
law  obtains  with  regard  to  both ;  namely,  between  like  ;>cu?m  there 
ff  rt-pu/siitn,  bfjiovn  inlikc  there  is  attraction 


MAGNETISM:.  301 

1111.  A  magnet,  whether  native  or  artificial   attracts  iron  or 
which  has  no  magnetic  properties  ;  but  it  both  attracts  and 

reptls  those  substances  when  they  are  magnetic  :  that  is  the 
oorth  pole  of  one  magnet  will  attract  the  south  pole  of  another, 
and  the  south  pole  of  one  will  attract  the  north  of  another  ; 
but  the  north  pole  of  the  one  repels  the  north  pole  of  the  other, 
and  the  south  pole  of  one  repels  the  south  pole  of  another. 

1112.  If  either  pole  of  a  magnet  be  brought  near  any  small 
piece  of  soft  iron,  it  will  attract  it.    Iron  filings  will  sdso  adhere 
in  clusters  to  either  pole. 

To  what    bod-        1113.    A.  magnet    *nay   communicate    its 

ies  are  the  ma  «•-  ,.  ,,  •.     ,     v 

netic  properties    properties   to   other    unmagnetized    bodies. 

most  easily  com-  But  these  properties  can  be  generally  con- 
municatcd?  •>  .  ,,  ,  . 

veyed   to    no   otter    substances    than   iron. 

nickel  or  cobalt,  without  the  aid  of  electricity. 

Coulomb  has  discovered  that  "  all  solid  belies  are  sus- 
ceptible of  magnetic  influence"  But  the  "  influence," 
is  perceptible  only  by  the  nicest  tests,  and  under  peculiar 
circumstances. 


What  are  per-      1H4.  All  permanent  natural  and  artificial 
manent  mag-   magnets,  as  well  as  the  bodies  on  wt  icli  they 
act,  are  either  iron  in  its  pure  state   or  such 
compounds  as  contain  it. 

What  effect  has  1115-  The  powers  of  a  magn  t  are  in- 
the  use  of  a  ma  g-  creased  by  action,  and  are  impaired  and 

net  on  its  -power?  1^.1.1          j- 

even  lost  by  long  disuse. 

TJ-,   .  .  1116.  When  the  two  poles  of  a  magnet  are 

V\  fiat  is  a 

horse-shoe  or  brought  together,  so  that  the  magnet  resembles 
u  ******  in  shape  a  horse-shoe,  or  the  capital  letter  U, 
it  is  called  a  horse-slioe  magnet,  or  a  U  magnet  ;  and  it  may 
be  made  to  sustain  a  considerable  weight,  by  suspending 
substances  from  a  small  iron  bar,  extending  from  one  pole 


302  NATURAL   PHILOSOPHY. 

to  tho  other.     This  bar  is  called  the  keeper.     A  small  adr- 
ditiui  may  be  made  to  the  weight  every  d&y. 

1117.  Soft  iron  acquires  the  magnetic  power  very  readily, 
And  also  loses  it  as  readily  ;  hardened  iron  or  steel  acquires 
the  property  with  difficulty,  b^t  retains  it  permanently. 

MTT.  <  f  77  1118.  When  a  magnet  is  broken  or  divided. 

What  follows 

when  a  mag-    each  part  becomes  a  perfect  magnet,   having 

ivide 


net  is  divided?  b()th  a  north  and  gouth  pole 

This  is  a  remarkable  circumstance,  since  the  central  part  of  a, 
magnet  appears  to  possess  but  little  of  the  magnetic  power; 
out,  when  a  magnet  is  divided  in  the  centre,  this  very  part  as- 
sumes the  magnetic  power,  and  becomes  possessed  in  the  one 
part  of  the  north,  and  in  the  other  of  the  south  polarity. 

1119.  The  magnetic  power  of  iron  or  steel  appears  to  reside 
wholly  on  the  surface,  and  is  independent  of  its  mass. 
In  what  do          1120.  In  this  respect  there  is  a  strong  resem- 

•magnetism  blance  between  magnetism  and  electricity.  Elec- 
and  electricity  .  .  A  ,  .  ;  ,  .. 

resemble  each   tricity,  as  has  already  been  stated,  is  wholly  con- 

other?  fined  to  the  surface  of  bodies.  In  a  few  words, 

magnetism  and  electricity  may  be  said  to  resemble  each  other 
in  the  following  particulars  : 

(U)  Each  consists  of  two  species,  namely,  the  vitreous  and 
the  resinous  (or,  the  positive  and  negative)  electricities  ;  and  the 
northern  or  southern  (sometimes  called  the  Boreal  and  the 
Austral)  polarity. 

(2.)  In  both  magnetism  and  electricity,  those  of  the  same 
aamc  repel,  and  those  of  different  names  attract  each  other. 

(3.)  The  laws  of  induction  in  both  are  similar. 

(4.)  The  influence,  in  both  cases  (as  has  just  been  stated) 
resides  at  the  surface,  and  is  wholly  independent  of  their  mass. 

What  effect        H21.  Heat  weakens,  and  a  great  degree  of 
has  heat  )n    heat  destroys  the  power  of  a  magnet  ;  but  the 
magnetic  attraction  is  undiminished  by  the  in- 
terposition of  any  bodies,  except  iron,  steel,  &c. 


MAGNETISM.  '60% 

ftiuai  jtiwr          1122.   Electricity    frequently    changes    the 

-'"uses  will  aj  •  poles  of  a  magnet ;  and  the  explosion  of  a  small 
feet  the  polar- 

ityofa  mag-  quantity  of  gunpowder,  on  one  of  the  poles. 
"d f  produces  the  same  effect.  Electricity,  also, 

sometimes  renders  iron  and  steel  magnetic,  which  were 
not  so  before  the  charge  was  received. 

What  is  the  1123.  The  effect  produced  by  two  magnets, 
efoutle°mag-  use(l  together,  is  much  more  than  double  that 
net  ?  of  either  one  used  alone. 

What  is  meant  1124.  When  a  magnet  is  suspended  freely 
by  "the  dip-  froin  fts  centre,  the  two  poles  will  not  lie  in 

ping  of  a  mag- 
net, and  hav  the  same  horizontal  direction.  This  is  called 
ts  it  corrected?  ^  jnciination  or  the  dipping  of  the  magnet. 
1125.  The  tendency  of  a  magnetic  needle  to  dip  is  corrected, 
in  the  mariner's  and  surveyor's  comp-asses,  by  making  the  south 
ends  of  the  needles  intended  for  use  in  northern  latitudes  some- 
what heavier  than  the  north  ends.  Compass-needles,  intended 
to  be  employed  on  long  voyages,  where  great  variations  of  lati- 
tude may  be  expected,  are  furnished  with  a  small  sliding-weight, 
by  the  adjusting  of  which  the  tendency  to  dip  may  be  counter- 
acted. The  cause  of  the  dipping  of  the  needle  is  the  superior 
attraction  caused  by  the  closer  proximity  of  the  pole  of  the  mag- 
net to  the  magnetic  pole  of  the  earth.  In  north  latitude,  the 
north  pole  of  the  needle  dips ;  in  south  latitude,  the  south  pole. 

ln  what  direc-      1126.  The  magnet,  when  suspended,  does  not 

Han  does  a       invariably  point  exactly  to  the  north  and  south 

nag-net  point         .          ,  .  ... 

when  free/y      points,  but  varies  a  little  towards  the  east  or 

•' l  Vended  >  the  west  This  variation  differs  at  different 
places,  at  different  seasons,  and  at  different  times  in  the  day. 
1127.  T?  e  variation  of  the  magnetic  needle  from  what  has  been 
supposed  its  true  polarity  was  a  phenomenon  that  for  centuries 
uad  baffled  the  science  of  the  philosopher  to  explain.  Recent 
discoveries  have  given  a  satisfactory  explanation  of  this  apparent 


54  NATURAL    PHILOSOPHY. 

anomaly.*  The  earth  has,  in  fact,  four  magnetic  poles,  two  of 
which  are  strong  and  two  are  weak.  The  strongest  north  pole 
is  in  America,  —  the  weakest,  in  Asia.  The  earth  itself  is  consid- 
ered as  a  magnet,  or,  rather,  as  composed  in  part  of  m^gnetie 
substances,  so  that  its  action  at  the  surface  is  irregular.  The 
variation  of  the  needle  from  the  true  geographical  meridian  ii 
therefore  subject  to  changes  more  or  less  irregular,  t 
What  gift  has  1128.  The  science  of  Magnetism  has  rendered 
Magnetism  immense  advantages  to  commerce  and  navigation, 
bestowed  on  by  means  of  the  mariner's  compass.  The  Mari- 

navigaticm  ?     ner's  Compass  consists  of  a  magnetized  bar  of  steel 
What  is  the         .     ,  „  .  fi       , 

Mariner's        called  a  needle;  having  at  its  centre  a  cap  fitted  to 

?       it,  which  is  supported  on  a   sharp-pointed  pivot 


*  The  following  statement  has  been  made  in  the  National  Intelligencer 
on  the  authority  of  its  London  correspondent  : 

Mr.  Faraday,  in  a  late  lecture  before  the  Royal  Institution  upon  the 
Magnetic  Forces,  made  the  following  important  announcement  . 

"  A  German  astronomer  has  for  many  years  been  watching  the  spots  on 
the  sun,  and  daily  recording  the  result.  From  year  to  year  the  groups  of 
spots  vary.  They  are  sometimes  very  numerous,  sometimes  they  are  few. 
After  a  while  it  became  evident  that  the  variation  in  number  followed  ?. 
descending  scale  through  five  years,  and  then  an  ascending  scale  through 
five  subsequent  years,  —  so  that  the  periodicity  of  the  variations  became  8 
visible  fact. 

"  While  our  German  friend  was  b»%jr  with  his  groups  of  sun-spots,  a* 
Englishman  was  busy  with  the  variations  of  the  magnetic  needle,  lie,  too. 
was  a  patient  recorder  of  patient  observation.  On  comparing  his  tabular 
results  with  those  of  the  German  astronomer,  he  found  that  the  variationi 
of  the  magnetic  ne>,Jle  corresponded  with  the  variations  of  the  sun-spots,  — 
that  the  years  when  the  groups  were  at  their  maximum,  the  variations  of 
the  needle  were  at  their  maximum,  and  so  on  through  their  series.  Thia 
relation  may  be  coincident  merely,  or  derivative  ;  if  the  latter,  then  do  we 
connect  astral  and  terrestrial  magnetism,  and  new  reaches  of  science  are 
open  to  us." 

t  The  northern  magnetic  pole  on  the  western  continent  is  in  latitude  70° 
N.  and  longitude  97°  VV.  On  the  eastern  continent  the  pole  is  about  at  the 
point  where  the  Lena  River  crosses  the  Arctic  circle.  The  south  poles  are 
nearly  on  the  Antarctic  circle,  one  in  130°  E.  longitude,  and  the  other  120° 
W.  from  Greenwich.  The  poles  are  doubtless  slowly  swinging  about  the 
poles  of  the  earth.  The  direction  of  the  needle  for  the  northeast  portion  of 
the  North  American  continent  is  west  of  north.  A  line  on  winch  the  needle 
points  clue  north  runs  through  Lake  Erie,  the  eastern  pint  of  Ohio,  a  c<  rner 
of  Pennsylvania,  the  District  of  Columbia,  and  North  Carolina.  West  of 
this  line  the  needle  points  to  the  e.ist  of  north.  At  San  Francisco  the  pres- 
ent direction  of  the  needle  (1871)  is  nearly  17°  east  of  north.  The  line  of 
no  variation  is  slowly  moving  westward,  and  the  direction  of  the  needle 
over  the  whole  continent  is  slowly  changing  in  the  same  direction. 


MAGNETISM. 


303 


Sxe<\  in  the  base  of  the  instrument.  A.  circular  plate,  or  card, 
the  circumference  of  which  is  divider  into  degrees,  is  attached 
to  the  needle,  and  turns  with  it.  On  an  inner  circle  of  the  card 
tho  thirty -two  points  of  the  mariner's  compass  are  inscribed 

Fig;  165. 


1129.  The  needle  is  generally  placed  under  the  card  of  a 
mariner's  compass,  so  that  it  is  out  of  sight;  but  small  needles, 
used  on  land,  are  placed  above  the  card,  not  attached  to  it,  and 
the  card  is  permanently  fixed  to  the  box. 

]  130.  The  compass  is  generally  fitted  by  two  sets  of  axes  to 
an  outer  box,  so  that  it  always  retains  a  horizontal  position, 
even  when  the  vessel  rolls.  When  the  artificial  magnet  or  necdU 
is  kept  thus  freely  suspended,  so  that  it  may  turn  north  or  south, 
the  pilot,  by  looking  at  its  position,  can  ascertain  in  what  direc- 
tion his  vessel  is  proceeding  j  and,  although  the  needle  varies  a 
little  from  a  correcjjrpolarity,  yet  this  variation  is  neither  so 
great,  nor  ro  irregular  as  seriously  to  impair  its  use  as  a  guide 
to  the  vessel  in  its  course  over  the  pathless  deep. 


JOO  N>.IUKAL    PHILOSOPHY 

1131.  The  invention  of  the  mariner's  ompass  is  usual  iv 
ascribed  to  Flavio  de  Melfi,  or  Flavio  Gioia,  a  Neapolitan,  about 
the  year  1302.  Some  authorities,  however,  assert  that  it  was 
brought  from  China  by  Marco  Paolo,  a  Venetian,  in  1260.  The 
invention  is  also  claimed  both  by  the  French  and  English. 

^  1132.  The  value  of  this  discovery  may  be  esti- 

*as  the  mar-  mated  from  the  consideration  that,  before  the  use 
.ncr's  com-  of  the  compass,  mariners  seldom  trusted  themselves 
out  of  sight  of  land ;  they  were  unable  to  make 
long  or  distant  voyages,  as  they  had  no  means  to  find  their  way 
back.  This  discovery  enabled  them  to  find  a  way  where  all  is 
trackless ;  to  conduct  their  vessels  through  the  mighty  ocean, 
out  of  the  sight  of  land  ;  and  to  prosecute  those  discoveries,  and 
perform  those  gallant  deeds,  which  have  immortalized  the  names 
of  Cook,  of  La  Perouse,  Vancouver,  Sir  Francis  Drake,  Nelson, 
Parry,  Franklin  and  others. 

Which  pole  of  1133.  Xhe  north  pole  of  a  magnet  is  more 
"ke^nore  *  powerful  in  the  northern  hemisphere,  or  north 
powerful  ?  of  the  equator,  and  the  south  pole  in  the  south- 
ern parts  of  the  world. 

1134.  When  a  piece  of  iron  is  brought  sufficiently  near  to  a 
magnet,  it  becomes  itself  a  magnet ;  and  bars  of  iron  that  have 
ptood  long  in  a  perpendicular  situation  are  generally  found  te 
be  magnetic. 

How  are  arti-  1135>  Artificial  magnets  are  made  by  ap- 
fieial  magnets  plying  one  or  more  powerful  magnets  to 
pieces  of  steel.  The  end  which  is  touched 
by  the  north  pole  becomes  the  south  pole  of  the  new  mag- 
net, and  that  touched  by  the  south  pole  becomes  the  north 
pole.  The  magnet  which  is  employed  in  magnetizing  a 
steel  bar  loses  none  of  its  power  by  being  thus  employed ; 
and  as  the  effect  is  increased  when  two  or  more  magnets 
are  used,  with  one  magnet  a  number  of  bars  may  be  mag- 
netized, and  then  combined  together;  by  which  means 


MAGNETISM.  307 

fcheir  power  may  be  indefinitely  increased.  Such  an  ap- 
paratus is  called  a  compound  magnet. 

1136.  There  are  several  methods  of  making  artificial  magnets. 
One  «f  the  most  simple  and  effectual  consists  in  passing  a  strong 
horse-shoe  magnet  over  bars  of  steel. 

In  making  bar  (or  straight)  magnets,  the  bars  must  be  laid 
lengthwise,  on  a  flat  table,  with  the  marked  end  of  one  bar 
against  the  unmarked  end  of  the  next ;  and  in  making  horse- 
shoe magnets,  the  pieces  of  steel,  previously  bent  into  their 
proper  form,  must  be  laid  with  their  ends  in  contact,  so  as  to 
form  a  figure  like  two  capital  U's,  with  their  tops  joined  together, 
thus,  c3£^ ;  observing  that  the  marked  ends  come  opposite  to 
those  which  are  not  marked ;  and  then,  in  either  case,  a  strong 
horse-shoe  magnet  is  to  be  passed,  with  moderate  pressure,  over 
the  bars,  taking  care  to  let  the  marked  end  of  this  magnet  pre- 
cede and  its  unmarked  end  follow  it,  and  to  move  it  constantly 
over  the  steel  bars,  so  as  to  enter  or  commence  the  process  at  a 
mark,  and  then  to  proceed  to  an  unmarked  end,  and  enter  the 
next  bar  at  its  marked  end,  and  so  proceed. 

After  having  thus  passed  over  the  bars  ten  or  a  dozen  times 
•jn  each  side,  and  in  the  same  direction  as  to  the  marks,  they 
will  be  converted  into  tolerably  strong  and  permanent  magnets. 
But  if,  after  having  continued  the  process  for  some  time,  the 
exciting  magnet  be  moved  over  the  bars  in  a  contrary  direc- 
tion, or  if  its  south  pole  should  be  permitted  to  precede  aftei 
the  north  pole  has  been  first  used,  the  previously-excited  mag- 
netism will  disappear,  and  the  bars  will  be  found  in  their  original 
state. 

This  mode  of  making  artificial  magnets  is  likely. to  be  wholly 
superseded  by  the  new  mode  by  electrical  aid  vthich  will  be 
noticed  in  connexion  with  Electro-magnetism. 

How  is  a  com-  1137-  A  compound  magnet  may  be  made  by 
pound  magnet  taking  several  horse-shoe  magnets  of  equal  size, 
constructed?  ^  after  liayillg  magnetized  them,  uniting  them 

together  by  means  of  screws. 
13* 


#08  NATURAL    PHILOSOPHY. 

1138.  A  magnetic  needle  is  made  by  fastening  tho  steel  on  a 
piece  of  ooard,  and  drawing  magnets  over  it  frjm  the  oentre 
outwards. 

1139.  A  horse-shoe   magnet  should   le    kepi 
How  should  a  7   r  ,,    ,  r  • 

horw-ihoe          armed,  by  a  small  bar  ol  iron  or  steel,  connect- 

magnet  be  kept  ?  ing    the   two    poles.      The  bar    is    called    "  the 
keeper" 

Interesting  experiments  may  be  made  by  a  magnet,  even  of  no 
great  power,  with  steel  or  iron  tilings,  email  needles,  pieces  of  fer- 
ruginous substances,  and  black  sand  which  contains  iron.  Such 
substances  may  be  made  to  assume  a  variety  of  amusing  forms  and 
positions  by  moving  the  magnet  under  the  card,  paper  or  t:»ble,  on 
which  they  are  placed.  Toys,  representing  fishes,  frogs,  aquatic 
birds,  &c.,  which  are  made  to  appear  to  bite  at  a  hook,  birds  floating 
on  the  water,  &c.,  are  constructed  on  magnetic  principles,  and  sold 
in  the  shops. 

What  is  Eke-       1140.   Electro-magnetism  relates  to  magnet- 
tro'-nagnetism?  jsm  which  js  induced  by  the  agency  of  electricity. 

1141.  The  passage  of  the  two  kinds  of  electricity  (namely,  the 
positive  and  the  negative)  through  their  circuit  is  called  the  elec- 
tric currents  ;  and  the  science  of  Electro-magnet' sin  explains  the 
phenomena  attending  those  currents.  It  has  already  been  stated 
that  from  the  connecting  wires  of  the  galvanic  circle,  or  battery, 
there  is  a  constant  current  of  electricity  passing  from  the  zinc  to 
the  copper,  and  from  the  copper  to  the  zinc  plates.  In  the  single 
circle  these  currents  will  be  negative  from  the  zinc,  and  positive 
from  the  copper  ;  but  in  the  compound  circles,  or  the  battery,  the 
current  of  positive  electricity  will  flow  from  the  zinc  to  the  copper, 
and  the  current  of  negative  electricity  from  the  copper  to  the  zinc. 
From  the  effect  produced  by  electricity  on  the  magnetic  needle,  it 
had  been  conjectured,  by  a  number  of  eminent  philosophers,  that 
magnetism,  or  magnetic  attraction,  is  in  some  manner  caused  b^ 
electricity.  In  the  year  1819,  Professor  (Ersted,  of  Copenhagen, 
made  the  grand  discovery  of  the  power  of  the  electric  current  to 
induce  magnetism  ;  thus  proving  the  connexion  between  magnetism 
and  electricity.  In  a  short  time  after  the  discovery  of  Professor 
(Ersted,  Mr.  Faraday  discovered  that  an  electrical  spark  could  be 
taken  from  a  magnet ;  and  thus  the  relation  between  magnetism 
and  electricity  was  fully  proved.  In  a  paper'published  a  few  years 
ago,  this  distinguished  philosopher  has  very  ably  maintained  ihe 
identity  of  common  electricity,  voltaic  electricity,  magnetic  electric- 
ity (or  electro-magnetism),  thermoelectricity,  and  animal  electric- 
ity. The  phenomena  exhibited  in  all  these  five  kinds  of  electricity 
(Jifler  merely  in  degree,  and  the  state  of  intensity  in  t'^c  action  cf  tlu; 


tti.&UTKO  -MAC  NkTIbM.  30;) 

fluid.  Thi  discovery  of  Professor  Oersted  has  been  followed  uut  by 
Ampere,  who,  by  his  mathematical  and  experimental  researches,  has 
presented  a  theory  of  the  science  less  obnoxious  to  objections  than 
that  proposed  by  the  professor.  The  discovery  of  CErstcd  was 
limited  to  the  action  of  the  electric  current  on  needles  previously 
magnetized ;  it  was  afterwards  ascertained  by  Sir  Humphrey  Davy 
and  M.  Arago  that  magnetism  maybe  developed  in  steel  not  pre- 
viously possessing  it,  if  the  steel  be  placed  in  the  electric  current. 
Both  of  these  philosophers,  independently  of  each  other,  ascertained 
that  the  uniting  wire,  becoming  a  magnet,  attracts  iron  filings  and 
collects  sufficient  to  acquire  the  diameter  of  a  common  quill  ;  but 
the  moment  the  connexion  is  broken,  all  the  filings  drop  off,  and  the 
attraction  diminishes  with  the  decaying  energy  of  the  pile.  Filings 
af  brass  or  copper,  or  wood-shavings,  are  not  attracted  at  all. 

1142.  All  the  effects  of  electricity  and  galvanism  that  have 
hitherto  been  described  have  been  produced  on  bodies  inter- 
posed between  the  extremities  of  conductors,  proceeding  from 
the  positive  and  negative  poles.  It  was  not  known,  until  the 
discoveries  of  Professor  (Ersted  were  made,  that  any  effect 
could  be  produced  when  the  electric  circuit  is  uninterrupted 

What  is  the  It  will  presently  be  seen  that  this  constitutes  the 
difference  be-  great  distinction  between  electricity  and  electro- 
\ricity  and  magnetism,  namely,  that  one  describes  the  effect 
electro-mag-  of  electricity  when  interrupted  in  its  course,  and 
that  the  other  more  especially  explains  the  effect  of 
an  uninterrupted  current  of  electricity. 

What  are  Jie       1143.  The  principal  facts  in  connexion  with  the 

prtnctpa          science  of  electro-magnetism  are,  — 
i  acts  of  elcc-  ° 

iro-magnet-         (1.)  That  the  electrical  current,  passing  uninter. 

tsm  ?  ruptedly  through  a  wire  connecting  the  two  ends 

of  a  galvanic  battery,  produces  an  effect  upon  the  magnetic 
needle. 

(2.)  That  electricity  will  induce  magnetism. 

(3.)  That  a  magnet,  or  bundle  of  magnets,  will  induce  elec- 
tricity. 

(4.)  That  the  combined  action  of  electricity  and  magnetism,  as 
described  in  this  science,  produces  a  rotatory  motion  of  certain 
kinds  of  bodies,  in  a  direction  pointed  out  by  certain  laws. 

(5.)   That  the  periodical  variation  of  the  magnetic   needle 


lilO  NATURAL   PHILOSOPHY. 

from  the  true  meridian,  or,  in  other  words,  the  variation  of  thf 
compass,  is  caused  by  the  influence  of  the  electric  currents. 

(6.)  That  the  magnetic  influence  is  not  confined  to  iron,  steel 
&c.,  but  that  most  metals,  and  many  other  substances,  may  be 
converted  into  temporary  magnets  by  electrical  action. 

(7.)  That  the  magnetic  attraction  of  iron,  steel,  &c.,  may  b« 
prodigiously  increased  by  electrical  agency. 

(8.)  That  the  direction  of  the  electric  current  may,  in  ah 
cases,  be  ascertained. 

(9.)  That  magnetism  is  produced  whenever  concentrated  elec- 
tricity is  passed  through  space. 

(10.)  That  whila  in  common  electrical  and  magnetic  attrac- 
tions and  repulsions  those  of  the  same  name  are  mutually 
repulsive,  and  those  of  different  names  attract  each  other,  in 
the  attractions  and  repulsions  of  electric,  currents  it  is  precisely 
the  reverse,  the  repulsion  taking  place  only  when  the  wires  are 
so  situated  that  the  currents  are  in  opposite  direction. 

The  consideration  of  the  subject  of  electricity  induced  by 
magnetism  properly  belongs  to  the  subject  of  Magneto-elec- 
tricity, in  which  connexion  it  will  be  particularly  noticed. 

How  is  the  1144.  The  direction  of  the  electric  current  is 

^rrenTo/0  ascertained  by  means  of  the  magnetic  needle.  If 
electricity  a  sheet  of  paper  be  placed  over  a  horse-shoe  mag- 
ascertained?  net>  an(j  fine  1,1^  sau(l,  or  steel  filings,  be  dropped 
loosely  on  the  paper,  the  particles  will  be  disposed  to  arrange 
themselves  in  a  regular  order,  and  in  the  direction  of  curve  lines. 
This  is,  undoubtedly,  the  eSect  of  some  influence,  whether  that 
ot  electricity,  or  of  magnetism  alone,  is  not  material  at  present 
to  decide. 

How  will  a  1145    A  magnet  freely  suspended  tench 

^magnTplace    to  assume  a  position  at  right  angles  to  the 

itself  in  relation    direction  of  a  current  of  electricity  passing 

to  the  electrical 

Current?  near  jt' 

11UJ.   If  a  wire,  which  connects  the  extremities  of  a  voltaie 


"*  LSM.  311 

Buttery,  be  brought  over  and  parallel  with  a  magnetic  neecTie  ut 
rest,  or  with  its  poles  properly  directed  north  and  south,  that 
end  of  the  needle  next  to  the  negative  pole  of  the  battery  wi.'l 
move  towards  the  west,  whether  the  wire  be  on  one  side  of  the 
needle  or  the  other,  provided  only  that  it  be  parallel  with  it. 

1147.  Again,  if  the  connecting  wire  be  lowered  on  either  side 
of  the  needle,  so  as  to  be  in  the  horizontal  plane  in  which  the 
needle  should  move,  it  will  not  move  in  that  plane,  but  will  have 
a  tendency  to  revolve  in  a  vertical  direction;  in  which,  however, 
it  will  be  prevented  from  moving,  in  consequence  of  tjie  attrac 
tion  of  the  earth,  and  the  manner  in  which  it  is  suspended 
When  the  wire  is  to  the  east  of  the  needle,  the  pole  nearest  to 
the  negative   extremity  of  the  battery  will  be  elevated ;  and 
when  it  is  on  the  west  side,  that  pole  will  be  depressed. 

1148.  If  the  connecting  wire  be  placed  below  the  plane  in 
which  the  needle  moves,  and  parallel  with  it,  the  pole  of  the 
needle  next  to  the  negative  end  of  the  wire  will  move  towaida 
the  east,  and  the  attractions  and  repulsions  will  be  the  reverse 
of  those  observed  in  the  former  case. 

How  does  the  1149.  The  action  of  the  conducting-wire  in 
ekctrt-magnetic  these  cases  exhibits  a  remarkable  peculiarity. 
current  act?  ^11  other  known  forceL  exerted  between  two 
points  act  in  the  direction  of  a  straight  line  connecting  these 
points,  and  such  is  the  case  with  electric  and  magnetic  actions, 
separately  considered;  but  the  electric  current  exerts  its  mag- 
netic influence  laterally,  at  right  angles  to  its  own  course.  Nor 
does  the  magnetic  pole  move  either  directly  towards  or  directly 
from  the  conducting-wire,  but  tends  to  revolve  around  it  without 
changing  its  distance.  Hence  the  force  must  be  considered  as 
acting  in  the  direction  of  a  tangent  to  the  circle  in  which  the 
magnetic  pole  would  move. 

What  effect  has        H5Q.  The  two  sides  of  an  unmagnetized 

a  voltaic  bat-  .  ,.  -i       i        •  i      ^i  ' 

f€ry  on  unrru.g-  stee^  needle  will  become  endued  with  the 
r«-m«/  «v»/.i  north  and  south  polaiity,  if  tlie  needle  be 


itti  NATURAL  PHILOSOPHY. 

placed  parallel  with  the  connecting  wire  of  a  voltaic  battery, 
and  nearly  or  quite  in  contact  with  it.  But,  if  the  needle 
be  placed  at  right  angles  with  the  connecting  wire,  it  will 
become  permanently  magnetic  ;  one  of  its  extremities  point- 
ing to  the  north  pole  and  the  other  to  the  south,  when  it  is 
finely  suspended  arid  suffered  to  vibrate  undisturbed. 
To  what  may  1151.  Magnetism  maybe  communicated 

Communicated      to    n'on   all(^  stee^    by  means   of  electricity 

by  the  voltaic,        from  an  electrical   machine ;  but  the  effect 

battery,  and  .  ,  .        .  . 

uhat  is  the  pro-    can  be  more  conveniently  produced  by  means 

cess  called  ?          Of  the  voltaic  battery.     This  phenomenon  is 

called  electro-magnetic  induction. 

What  is  a        1152.  A  Helix  is  a  spiral  line,  or  a  line  wound 

Helix  ?        into  the  shape  of  a  cork-screw. 

What  use  is  H53.  If  a  helix  be  formed  of  wire,  and  a 

made  of  a  helix     ,  „          ,  ,  ,        ,      .  ,  .        ,      .    . . 

in  conne.rhm  "ar  °*  stee^  "e  enclosed  within  the  helix,  on 
irith  the  battery  ?  applying  the  conducting- wires  of  the  battery 
to  the  extremities  of  the  helix,  the  steel  bar  will  immediately 
become  magnetic.  The  electricity  from  a  common  electrical 
machine,  when  passed  through  the  helix,  will  produce  the 
same  effect. 

The  wire  which  forms  the  helix  should 


And  what  must 

first  be  done  be  coated  with  some  non-conducting  substance, 
uitfi  the  wire  of  Slich  as  silk  wound  around  it :  as  it  may  then 
the  helix  *  in  •  M  n-  -  i 

be  formed  into  close  coils,  without  suffering  trio 

electric  fluids  to  pass  from  surface  to  surface,  which  would  im- 
pair its  effect. 

1155.  If  such  a  helix  be  so   placed  that  it  may  move  freely 
as  when  made  to  float  on  a  basin  of  water,  it  will  be  attracted 
and  repelled  by  the  opposite  poles  of  a  common  magnet. 

1156.  If  a  magnetic  needle  be  surrounded  by  coiled  wire. 
covered  with  silk,  a  very  minute  portion  of  electricity  through 


ELECT K<  >- M  A  (T«  fcTISM .  3 1  it 

the  wire  will  cause  the  needle  to  deviate  from  it.s  proper 
lirectiori. 

Wh  tisar  Elec  H&7.  A  needle  thus  prepared  is  called  an 
ro-mag-nf.tic  Electro-magnetic  Multiplier.  It  is,  ia  fact,  a 
Muliipier  very  delicate  electroscope,  or  rather  galvanom- 

eter, capable  of  pointing  out  the  direction  of  the  electric  cur 
rent  in  all  cases. 

1158.   Among  the  most  remarkable  of  the 
What  is  meant     „     A  ,      ..,     ,,  .  0     , 

•)>/  the  Electro-    ^acts  connected  with  the  science  of  electro- 

magnetic  Rota-  magnetism  is  what  is  called  the  Electro- 
magnetic Rotation.  Any  wire  through 
which  a  current  of  electricity  is  passing  has  a  tendency  to 
revolve  around  a  magnetic  pole  in  a  plane  perpendicular  to 
the  current,  and  that  without  reference  to  the  axis  of  the 
magnet  the  pole  of  which  is  used.  In  like  manner  a  mag 
netic  pole  has  a  tendency  to  revolve  around  such  a  wire. 

1159.  Suppose  the  wire  perpendicular,  its  upper  end  posi- 
tive, or  attached  to  the  positive  pole  of  the  voltaic  battery,  and 
its  lower  end  negative ;  and  let  the  centre  of  a  watch-dial  rep- 
resent the  magnetic  pole :  if  it  be  a  north  pole,  the  wire  will 
rotate  round  it  in  the  direction  that  the  hands  move ;  if  it  be  a 
south  pole,  the  motion  will  be  in  the  opposite  direction.  From 
these  two,  the  motions  which  would  take  place  if  the  wire  were 
inverted,  or  the  pole  changed,  or  made  to  move,  may  be  readily 
ascertained,  since  the  relation  now  pointed  out  remains  constant. 

1160.  Fig.  166  represents  the  ingenious  ap- 
^XVM  i  Fig.  paratus>  invented  by  Mr.  Faraday,  to  illustrate 
the  electro-magnetic  rotation.  The  central  pil- 
.ar  supports  a  piece  of  thick  copper  wire,  which,  on  the  one 
side,  dips  into  the  mercury  contained  in  a  small  glass  cujr  'A 
To  a  pin  at  the  bottom  of  this  cup  a  small  cylindrical  magnet 
is  attached  by  a  thread,  so  that  one  pole  shall  rise  a  little  above 
i.he  surface  of  the  mercury,  and  be  at  liberty  to  move  around 
Mi<!  wire.  Tlu>  bottom  of  the  cup  is  perforate!,  nnd  hup  a  cop* 


NATURAL    HHCLOSOPliY. 


16Q- 


per  pin  passing  through  it,  which,  touching  the  mercury  on  the 
inside,  is  also  in  contact  with  the  wire  that  proceeds  outward?. 
'  n  that  side  of  the  in- 
i  .{.rumen t.  On  the  other 
bide  of  the  instrument  t>, 
the  thick  copper  wire, 
soon  after  turning  down, 
terminates,  but  a  thinnci 
piece  of  wire  forms  a 
communication  between 
it  and  the  mercury  on 
the  cup  beneath.  As 
freedom  of  motion  is  re- 
garded in  the  wire,  it  is  made  to  communicate  with  the  formci 
by  a  ball  and  socket-joint,  the  ball  being  held  in  the  socket  by 
a  thread  ;  or  else  the  ends  are  bent  into  hooks,  and  the  one  is 
then  hooked  to  the  other.  As  good  metallic  contact  is  required, 
the  parts  should  be  amalgamated,  and  a  small  drop  of  mercury 
placed  between  them;  the  lower  ends  of  the  wire  should  also  be 
amalgamated.  Beneath  the  hanging  wire  a  small  circular  mag- 
net is  fixed  in  the  socket  of  the  cup  £,  so  that  one  of  its  poles 
is  a  little  above  the  mercury.  As  in  the  former  cup,  a  metallic 
connexion  is  made  through  the  bottom,  from  the  mercury  to  the 
external  wire. 

If  now  the  poles  of  a  battery  be  connected  with  the  horizon- 
tal external  wires  c  c,  the  current  of  electricity  will  be  through 
the  mercury  and  the  horizontal  wire,  on  the  pillar  which  con 
nects  them,  and  it  will  now  be  found  that  the  movable  part  of 
the  wire  will  rotate  around  the  magnetic  pole  in  the  cup  b,  and 
the  magnetic  pole  round  the  fixed  wire  in  the  other  cup  a,  in  the 
direction  before  mentioned. 

By  u-.ing  a  very  delicate  apparatus,  the  magnetic  pole  oi  the 
earth  may  be  made  to  put  the  wire  in  motion. 

Kjrtfoiii    Pi*  1161.  Fig.   167  represents  another  ingenious 

Ki<.  contrivance,  invented  by  M.  Ampere   for  illus- 


ELECTKO-MAGNETISK.  315 

tratiiig  the  electro-magnetic  rotation  ;  and  it  has  the  advantage 
of  comprising  within  itself  the  voltaic  combination  which  is 
employed.  It  consists  of  a  cylinder  of  copper 
about  two  inches  high,  and  a  little  less  than  two 
inches  internal  diameter,  within  which  is  a  small 
cylinder,  about  one  inch  in  diameter.  The  two 
cylinders  are  connected  together  by  a  bottom, 
having  an  aperture  in  its  centre  the  size  of  the 
smaller  cylinder,  leaving  a  circular  cell,  which 
may  be  filled  with  acid.  A  piece  of  strong  cop- 
per wire  is  fastened  across  the  top  of  the  inner 
cylinder,  and  from  the  middle  of  it  rises,  at  a 
right  angle,  a  piece  of  copper  wire,  supporting  a 
very  small  metal  cup,  containing  a  few  globules  of  mercury.  A 
cylinder  of  zinc,  open  at  each  end,  and  about  an  inch  and  a 
quarter  in  diameter,  completes  the  voltaic  combination.  To  the 
latter  cylinder  a  wire,  bent  like  an  inverted  U,  is  soldered  at 
opposite  sides ;  and  in  the  bend  of  this  wire  a  metallic  point  is 
fixed,  which,  when  inserted  in  the  little  cup  of  mercury,  sus- 
pends the  zinc  cylinder  in  the  cell,  and  allows  it  a  free  circular 
motion.  An  additional  point  is  directed  downwards  from  the 
contra!  part  of  the  stronger  wire,  which  point  is  adapted  to  a 
small  hole  at  the  top  of  a  powerful  bar  magnet.  When  the 
apparatus  with  one  point  only  is  charged  with  diluted  acid,  and 
set  on  the  magnet  placed  vertically,  the  zinc  cylinder  revolves 
'•n  a  direction  determined  by  the  magnetip  pole  which  is  upper- 
most. With  two  points,  the  copper  revolves  in  one  direction 
and  the  zinc  in  a  contrary  direction. 

1162.  If,  instead  of  a  bar  magnet,  a  horse-shoe  magnet  be 
employed,  with  an  apparatus  on  each  pole  similar  to  that  which 
has  now  been  described,  the  cylinders  in  each  will  revolve  in 
apposite  directions.  The  small  cups  of  mercury  mentioned  in 
the  preceding  description  are  sometimes  omitted,  and  the  points 
are  inserted  in  an  indentation  on  the  inverted  U. 


NATURAL    PHILOSOPHY. 


How  is  the  mag  1163.  The  magnetizing  power  of  the  con- 
o/lhenKryr  ducting  »*»  of  a  Battery  is  very  greatly  in- 
increased  f  creased  by  coiling  it  into  a  helix,  into  which 
the  body  to  be  magnetized  may  be  inserted.  A  single  cir- 
cular turn  is  more  efficient  than  a  straight  wire,  and  each 
turn  adds  to  the  power  within  a  certain  limit,  whether  the 
whole  forms  a  single  layer,  or  whether  each  successive  turn 
encloses  the  previous  one.* 

How  is  a  Mix  1164'  When  a  helix  of  Sreat  P°wer  is 
of  great  power  required,  it  is  composed  of  several  layers  of 

obtained?  .         mu         •**"«.          -i  t  v 

wire.     Ihe  wire  terming  the  coil  must  be 

insulated  by  being  wound  with  cotton,  to  prevent  any  lat- 
eral passage  of  the  current. 

1165.  Fig.  168  represents  a  helix  on  a  stand. 
A  bar  of  soft  iron,  N  S,  being  placed  within 
tbe  helix,  is  connected  with  the  battery-  bv 

Fig.  168. 


Explain 
Fig.  168. 


*  The  amount  of  power  dereloped  by  electro-magnets  depends,  when  the 
size  of  the  magnet  and  its  envelope  of  wire  is  suitable,  upon  the  amount  of 
zinc  consumed  in  the  battery.  Much  controversy  has  been  held  about  the 
possibility  of  so  employing  electro-magnetism  as  to  substitute  it  economic- 
ally as  a  motive  force  for  those  agents  now  employed — viz.,  steam,  water, 
and  air.  The  attempts  thus  far  have  been,  unsuccessful.  Although  many 
machines  have  been  devised  whose  propelling  force  was  electricity  or  olec- 
tro-magnetiRm,  the  cost  of  their  propulsion  was  far  greater  than  by  any 
other  plan  in  use. 


ELE.OTKO-MAGNETISM.  H  1  7 

tncans  of  the  screw-cups  on  the  base  of  the  stand.  The  two 
extremities  of  the  bar  instantly  become  strongly  magnetic,  and 
keys,  or  pieces  of  iron,  iron  filings,  nails,  &c.,  will  be  held  up 
so  Jong  as  the  connexion  with  the  battery  is  sustained.  But,  so 
soon  as  the  connexion  is  broken,  the  bar  loses  its  magnetic 
power,  and  the  suspended  articles  will  fall.  The  bar  can  be 
made  alternately  to  take  up  and  drop  such  magnetizable  articles 
as  are  brought  near  it,  as  the  connexion  with  the  battery  is 
made  or  broken. 

1166.  A  steel  bar  placed  within  the  helix  acquires  the  polar- 
ity less  readily,  but  retains  it  after  the  connexion  is  broken. 
Small  rods  or  bars  of  steel,  needles,  &c.,  may  be  made  perma 
nent  magnets  in  this  way. 

1167.  A  bar  temporarily  magnetized  bv 
What  is  an  Etec-     .         ,     ,   .  .    .          n    ,  r,, 
tro-ma'rnet?         "ie   electric   current   is    called  an  Electro- 
magnet. 

1168.  To  ascertain  the  poles  of  an  elec- 

ffow  ran  the  poles  .  .          ,  ,      ,     J       , 

of  an  electro-        tro-magnet,  it   must   be  observed  that  the 
magnet   be   dis-  north  pole  will  be  at  the  furthest  end  of 
the  helix  when  the  current  circulates  in 
the  direction  of  the  hands  of  a  watch. 

1169  Magnets  of  prodigious  power  have  been  formed  by 
mean-1  of  voltaic  electricity. 

\Vliat  was  the  H70.  An  electro-magnet  was  constructed  by 
ptnt-KT  of  the  Professor  Henry  and  Dr.  Ten  Eyck  which  was 

electro-magnets     capal)]e  Of  supporting  a  weight  of  750  pounds. 

constructed    .v/         L 

Prof.  Henry        They   have  subsequently    constructed    another, 

and ^  Dr.  Ten  which  will  sustain  2063  pounds.  It  consists  of 
a  bar  of  soft  iron,  bent  into  the  form  of  2 
horse-shoe,  and  wound  with  twenty-six  strands  of  copper  bell- 
wire,  covered  with  cotton  threads,  each  thirty-one  feet  long ; 
about  eighteen  inches  of  the  ends  are  left  projecting,  so  that 
only  twenty-eight  feet  of  each  actually  surround  the  iron.  The 
aggregate  length  of  the  coils  is  therefore  728  feet.  Each  strand 


NATURAL    PHILOSOPHY. 

is  ^ound  on  a  little  less  than  an  inch ;  in  the  middle  of  the 
horse-shoe  it  forms  three  thicknesses  of  wire ;  and  on  tho  ends, 
or  near  the  poles,  it  is  wound  so  as  to  form  six  thicknesses.  Bo 
ing  connected  with  a  battery  consisting  of  plates,  containing  u 
little  less  than  forty-eight  square  feet  of  surface,  the  ipagnet 
supported  the  prodigious  weight  stated  above,  namely,  2063 
pounds. 

1171.  HE-  Fig-  169. 

Explain  Fig.       LIACAL  RlNO> 

Fig.  169  rep- 
resents a  heliacal  ring,  or  ring 
of  wire  bent  in  the  form  of  a 
helix,  with  the  ends  of  the  * 
wire  left  free  to  be  inserted 
in  the  screw-cups  of  a  bat- 
tery. Two  semicircular  pieces 
of  soft  unmagnetized  iron, 
furnished  with  rings,  —  the 
upper  one  for  the  hand,  the 
lower  one  for  weights, — 
tire  prepared  to  be  inserted 
into  the  helix,  in  the  manner 
of  the  links  of  a  chain.  As 
soon  as  the  ends  of  the  helix 
are  inserted  into  the  screw- 
cups  of  the  battery,  the  rings  will  be  held  together,  with  gre;;t 
force,  by  magnetic  attraction. 

1172.  That  the  attraction  is  caused,  or  that  the  magnetism  is  in- 
duced, by  the  circulation  of  electricity  around  the  coils,  may  be 
proved  by  the  following  interesting  experiment.  Hold  the  heliacal 
ring  horizontally  over  a  plate  of  small  nails,  and  suspend  an  unmag- 
netized bar  perpendiculai  ly  on  the  outside  of  the  ring,  over  the  nails, 
and  there  will  be  no  attraction.  Suspend  the  bar  perpendicularly 
through  the  helix,  and  the  nails  will  all  attach  themselves  to  it  in 
the  form  of  tangents  to  the  circles  formed  by  the  coils  of  the  helia 
cal  ring. 

H  ehors&-  1-173.  Communication  of  Magnetism  to  Steel 
shoe  magnets  by  tfie  Electro-magnet. —  Bars  of  the  U  fora? 


ELECTKO-MAGNET1G    TELEGKAPH . 


most  readily  are  most  readily  magnetized  by  drawing  them 
from  the  bend  to  the  extremities,  across  the 
poles  of  the  U  electro-magnet,  in  such  a  way  that  both  halves 
of  the  bar  may  pass  at  the  same  time  over  the  poles  to  which 
they  are  applied.  This  should  be  repeated  several  times,  recol- 
lecting always  to  draw  the  bar  in  the  same  direction. 

1174.  Fig.  170  represents  the  U  electro- 
magnet with  the  bar  to  be  magnetized.  Whec 
the  bar  is  thick,  both  surfaces  should  be  drawn 
across  the  electro-magnet,  keeping  each  half  applied  to  the 
same  pole.  To  remove  the  magnetism,  it  is  only  necessary  to 

Fig.  170. 


Explain   Fig. 
1TO. 


On  what  funda- 
mental principle 
is  the  Electric 
Telegraph  con- 
constructed  1 


reverse  the  process  by  which  it  was  magnetized,  that  is,  to  draw 
the  bar  across  the  electro-magnet  in  a  contrary  direction. 

1175.  THE  ELECTRO-MAGNETIC  TELEGRAPH.* 
—  From  the  description  which  has  now  been 
given  of  the  electro-magnetic  power,  it  will 
readily  be  perceived  that  a  great  force  can  be 
made  to  act  simply  by  bringing  a  wire  into 
contact  with  another  conductor,  and  that  the  force  can  be  in- 
stantly arrested  in  its  operation  by  removing  the  wire  from  the 
contact ;  in  other  words,  that  by  connecting  and  disconnecting 
a  helix  with  a  battery,  a  prodigious  power  can  be  made  succes 

*  The  word  telegraph  is  compounded  of  two  Greek  words,  rtfts  (tele),  sig- 
nifying at  a  distance,  and  ynayo  (grapho),  to  write,  that  is,  to  signify  or  to 
write  at  a  distance.  The  word  telescope  is  another  compound  of  the  word 
fijit  with  the  word  ox^nia:  (scrpio),  to  see, —  an  instrument  to  see  at  a  di* 
tartce. 


B"0  NATURAL    PHILOS01  IIY 

eively  to  act  and  cease  to  act.     Advantage  has  been  taktii  o' 
this  principle  in  the  construction  of  the  American  electro-mag; 
netic   telegraph,  which  was  matured  by  Professor  Morse,  and 
first  put    into  operation  between  the   cities  of  Baltimore  and 
.  Washington,  in  1844>     It  was  not,  however, 

ies  'rendereTl-he  unti*  Professor  Henry,  of  Princeton,  New  Jer- 
magnetic  tele-  sey,  had  discovered  the  mode  of  constructing 
tfraph  possible?  the  powerful  electro-magnets  which  have  been 
noticed,  that  this  form  of  the  telegraph  became  possible. 

1176.  The  principles  of  its  construction  may 
Explain  the  man-  b    b  {  fl  d        fon  ^. 

ner  m  which  the  J 

electric  telegraph  An  electro-magnet  is  so  arranged  with  its 
performs  its  armature  that  when  the  armature  is  attracted 

it  communicates  its  motion  to  a  lever,  to  which 
a  blunt  point  is  attached,  which  marks  a  narrow  strip  of  paper, 
drawn  under  it  by  machinery  resembling  clock-work,  whenever 
the  electro-magnet  is  in  action.  When  the  electro-magnet  ceases 
to  act,  the  armature  falls,  arid,  communicating  its  motion  to  the 
lever,  the  blunt  point  is  removed  from  its  contact  with  the  paper. 
By  this  means,  if  one  of  the  wires  from  the  battery  is  attached 
to  the  screw-cup,  whenever  the  other  wire  is  attached  to  the 
remaining  cup  the  armature  is  powerfully  attracted  by  the 
magnet,  and  the  point  on  the  lever  presses  the  paper  into  the 
groove,  of  a  roller,  thereby  making  an  indentation  on  the  paper, 
corresponding  in  length  to  the  time  during  which  the  contact 
with  the  battery  is  maintained,  the  paper  being  drawn  slowly 
under  the  roller. 

1177.   In  the  construction  of  the  electric  tele- 
\Vhat  is  the 
agent  in  the       graph  the  first  object  of  consideration  is  the  de- 

electrti  tele-        yelopment  of  the  agent.    The  agent  is  the  electric 

fluid,  which  is  brought  into  action  by  a  battery. 
The  forms  of  batteries  chiefly  employed  for  this  purpose  are 
Grove's,  Bunsen's,  Daniel's,  Snell's  or  its  modifications,  and  Le- 
clanche's.  If  the  telegraph  is  in  constant  use,  and  the  battery  is 
to  be  renewed  at  intervals  of  two  or  three  days,  one  of  the  first 
above  mentioned  is  used. 


EJ,KCTKO-MAGNETIO    TELEGKAPH. 


Fig.    lYl   represents   a  battery    composed    of 
ffxplatn  Fig.    twejve  Cllps>  on  t^e  princip]e  of  Grove's  battery, 

each  cup  containing  a  thick  cylinder  of  zinc, 
with  a  porous  cell,  two  acids,  and  a  strip  of  platinum,  as 
described  in  Fijr.  104.  The  chemical  action  of  the  acids  on  the 


Fig.  171 


Fig.  172. 


zinc  generates  a  powerful  current   of  electricity    towards  the 
screw-cups  at  A  B. 

1178.  The  second  step  in  the  construction  of 
Explain  Fig.  ^  telegraph  is  represcntcd  by  Fig.  172.  The 
wires  from  the  battery  represented  in  Fig.  171 
are  carried  to  the  screw-cups  in  the  apparatus  represented  by 
Fig.  172,  called  the  sig- 
nal-key, A  to  A  and  B 
to  B,  respectively.  It 
will  be  observed  that  the 
cups  of  the  signal-key  are 
insulated,  and  that  the 
electric  fluid  can  finish  its 
circuit  only  when  the  fin- 
ger depresses  the  knob 
and  makes  it  come  in  con- 
tact with  the  metallic  strip  below,  thus  forming  a  communication 
between  the  screw-cups.  The  signal -key  thus  regulates  the  com- 
pletion of  the  circuit,  and  the  flow  of  the  current  of  electricity, 
ut  the  will  of  the  operator. 


822 


NA1UKAL   PHILOSOPHY. 


1179.  The   signal-key   is   made    in    several 
Explain  Fig.       forms  in  the  different  telegraphs,  and  in  Fig. 

173  is  represented  in  its  more  perfect  construc- 
tion.    It  coir  Ists  of  a  lever,  mounted  on  a  horizontal  axis,  with 
a,  knob  of  ivory  for  the  hand  at  the  extremity  of  the  long  arm. 
wrhich  is  at  the   left  in 
the  cut.     This    lever  is 
thrown  up  by  a  spring, 
so  as   to    avoid  contact 
with  the  button  on   the 
frame  below,  except  when 
the  lever  is  depressed  for 
the    purpose     of     com- 
pleting the  circuit.    A  regulating  screw  is  seen  at  the  extremity 
of  the  short  arm  of  the  lever,  which  graduates  precisely  the 
amount  of  motion  of  which  it  is  at  any  time  susceptible. 

1180.  The  third  and  last  part  of  the  tele- 
Explain   Fig.      graph  is  the  registering  apparatus,  represented 

in  Fig.  174. 

Here  are  two  screw-cups,  for  the  insertion  of  the  wires  from 
i  distant  battery.     An  iron  in  the  shape  of  a  U  magnet  stands 

Fig    174 


at  the  left  of  the  screw-cups,  each  arm  of  which  is  surrounded 
by  a  helix  or  coil  of  wire,  the  ends-  of  which,  passing  down  through 
the  stand,  are  connected  below  with  the  screw-cups.  It  will 
then  be  seen  that  when  the  signal-key  is  depressed  the  electrir 
circuit  is  completed,  and  that  the  electricity,  passing  through 
fhe  noils  of  wire,  renders  the  U-shaped  iron  highly  magnetic 


iU JSUTKO- UAUNJET  1C    TELL  ./  .*A  PH. 
Fig.  17fc. 


321  NATURAL    PHILOSOPHY. 

and  it  attracts  the  armature  down.  The  armature  IK  fixed  to 
the  shorter  arm  of  a  lever,  and  when  the  shorter  arm  is  attracted 
down,  the  longer  arm,  with  a  point  affixed,  is  forced  upward  and 
makes  an  indentation  upon  a  strip  of  paper.  The  length  of  the 
indentation  on  the  paper  will  depend  on  the  length  of  time  that 
the  signal-key  is  depressed.  When  the  signal-key  is  permitted 
again  to  rise,  the  electric  current  is  broken,  the  U-shaped  iron 
ceases  to  be  a  magnet,  and,  the  armature  being  no  longer 
attracted,  the  weight  of  the  longer  arm  will  cause  that  arm  to 
fall,  and  no  mark  is  made  on  the  paper. 

When  the  telegraph  was  first  constructed,  it  was  thought  nec- 
essary to  have  two  wires  in  order  to  form  the  circuit.  It  has 
since  been  found  that  the  earth  itself  will  serve  for  one-half  the 
circuit,  and  that  one  wire  will  alone  be  necessary  to  perform  the 
work  of  the  telegraph. 

1180.  Fig.  175  represents  the  manner  in  which 
Explain  Fig.  tne  e]ectric  telegraph  is  put  into  operation.  -On 
the  left  of  the  figure  is  seen  the  operator,  with 
the  battery  at  his  feet  and  his  finger  on  the  signal-key.  From 
one  screw-cup  of  the  battery  extends  a  wire  which  traverses 
the  whole  distance  between  two  cities,  elevated  on  posts  for 
security.  In  the  distant  city  the  wire  reaches  another  screw- 
cup  to  which  it  is  attached,  while  from  another  screw-cup  at 'the 
same  station  another  wire  is  attached,  which  extends  back  to  the 
operator  first  mentioned.  The  depression  of  the  signal-key 
forms  a  connexion  between  the  two  poles  of  the  battery  by 
means  of  the  wire,  and  the  fluid  will  traverse  the  whole  distance 
between  the  two  stations  in  preference  to  leaping  over  the  space 
between  the  two  screw-cups.  The  right  of  the  figure  represents 
the  receiver  of  the  information,  reading  the  message  which  has 
thus  been  imprinted  by  the  point. 

1182.  In  the  preceding  figures  the  mere  out- 

ExpUun  fig.      j-neg  ^ave  heen  given,  in  order  that  they  may 

be  distinctly  understood.     To  present  the  strip 

of  paoer   bo  that   it  may  readily  receive  the  impression,  addi- 


ELECTRO-MAGNETIC   TELEGRAPH. 


325 


tional  machinery  becomes  necessary.     The  complete  registering 
machine  is  shown  in  Fig.  176,  in  which  S  represents  a  large  spool 

Pig.  176. 


on  which  the  paper  is  wound,  and  clock-work  with  rollers  to 
give  the  paper  a  steady  motion  toward  the  point  by  which  the 
marks  are  to  be  made.  A  bell  is  sometimes  added,  which  is 
struck  by  a  hammer  when  the  lever  first  begins  to  move,  in  order 
to  draw  the  attention  of  the  operator. 

1183.  .It  will  be  recollected  that  this  form  of  the  magnetic 
telegraph  is  familiarly  known  as  Morse's,  the  machine  making 
nothing  but  straight  marks  on  the  slip  of  paper.  But  these 
straight  marks  may  be  made  long  or  short,  at  the  pleasure  of  the 
operator.  If  the  key  be  pressed  down  and  instantly  be  per- 
mitted to  rise,  it  will  make  a  short  line,  not  longer  than  a 
hyphen.  By  means  of  a  conventional  alphabet,  in  wJijch  the 
tetters  are  expressed  by  the  repetition  and  combination  of  marks 
varying  in  length,  any  message  may  be  conveniently  spelt  out, 
so  as  to  be  distinctly  understood  at  the  distant  station.  Thesa 
are  the  essential  features  of  Morse's  Telegraph. 


ISO  NATURAL    PHILOSOPHY. 

1184.  It  is  necessary,  in  long  lines  of  telegraphs,  to  combine  tlia 
efiects  of  several  batteries  to  supply  the  loss  of  power  in  traversing 
long  circuits.     This  is  done  by  local  batteries  or  relays,  as  they  are 
sometimes  called,  familiarly  known  in  connexion  with  Morse's  tele- 
graph.    The  use  of  the  relays  may  be  dispensed  with  by  increasing 
the  power  of  the  batery,  or  distributing  it  in  groups  along  the  line 
It  is  sometimes  divided  by  arranging  one-half  at  each  end  of  the 
line      For  every  twenty  miles  an  addition  of  one  of  Grove's  pint 
sups  should  be  made.     The  expense  of  acids  for  each  cup  for  two 
days   does   not   much    exceed   one  cent.     For  a  line  of  telegraph 
extending  around  the  earth,  twelve  hundred  Grove's  cups  would  be 
required,  distributed  at  equal  distances,  fifty  in  a  group. 

1185.  BAIN'S    TELEGRAPH.  —  The   telegraph   known   by   the 
name  of  Bain's  telegraph,  the  simplest  now  in  use,  differs  from 
Morse's  principally  in  its  mode  of  registering.     It  performs  its 
work  by  the  decomposition  of  a  saline  solution.     The  pen  or 
point  is  stationary.     A  circular  tablet,  moved  by  clock-work, 
under  the  point,  receives  the  point  in  concentric  grooves,  and 
the  writing  is  arranged  in  spiral    lines,  occupying   but  little 
space. 

Explain  Fig.  177  represents  Bain's  telegraph.  The  pen 
Fig.  177.  holder  is  connected  with  the  positive  wire  of  the 
battery,  and  the  tablet  with  the  negative.  The  circuit  is  COIB« 

Fig.  177. 


ELECTROMAGNETIC    TELEGRAPH. 


Fig.  178. 


Dieted  by  paper  moistened  with  a  solution  of  the  yellow  prus- 
siate  of  potash,  acidulated  with  nitric  or  sulphuric  acid.  The 
pen-wire  is  of  iron.  When  the  circuit  is  completed,  the  solution 
attacks  the  pen,  dissolves  a  portion  of  its  iron,  and  forms  the 
color  known  as  Prussian  blue,  which  stains  the  paper.  The 
alphabet  used  by  this  line  is  the  same  in  principle  as  that  used 
m  the  telegraph  of  Morse.  The  advantage  of  this  telegraph 
consists  in^  the  rapidity  with  which  the  disks  at  both  ends  are 
made  to  revolve,  by  which  a  message  may  be  communicated  at 
the  rate  of  a  thousand  letters  in  a  minute. 

Explain  H86.  The 
Fig.  178.  CCH  commonly 
used  in  connexion  with 
Bain's  telegraph  is  rep- 
resented in  Fig.  178.  It 
consists  of  a  U  magnet, 
each  arm  surrounded  by 
a  helix  of  wires,  which, 
when  the  current  passes, 
causes  the  armature  to 
be  attracted  and  give  mo- 
tion to  machinery,  by 
which  a  bell  or  a  glass  is 
rung. 

Explain          1187.     Fig. 

rig.  179.  179  represents  the  receiving  magnet  in  its  improved 
form.  The  armature  is 
mounted  on  an  upright 
bar,  directly  before  the 
poles  of  the  U  magnet, 
which  is  surrounded  by 
many  coils  of  insulated 
wire.  In  this  magnet 
the  points  of  contact  are 
preserved  from  oxidation 
by  the  use  of  platinum. 


Fig.  179. 


3z8  NATURAL    PHILOSOPHY. 

1188.  HOUSE'S  PRINTING  TELEGRAPH.  — This  telegraph  differ? 
from  the  other  principally  in  its  printing  with  great  rapidity 
the  letters  which  form  the  message. 

Explain  1189.  Fig.  180  represents  the  mechanical  part  of 
Fig.  180.  House's  telegraph.  The  operator  sits  at  a  key-board 
gimilar  to  that  of  a  pianoforte  or  organ,  and,  by  depressing  a 


Vig.  180. 


key,  the  letter  corresponding  with  the  key  ie  made  to  appear  at 
a  little  window  at  the  top  of  the  instrument,  while  it  is  at  the 
same  time  printed  on  a  strip  of  paper  below.  The  principle  by 
which  this  exceedingly  ingenious  operation  is  performed  is 
simply  this :  A  given  number  of  electrical  impulses  are  given 
for  each  letter.  These  impulses  give  motion  to  a  wheel,  so  that 
on  the  depression  of  a  key  the  circuit  will  be  broken  at  precisely 
the  point  which  corresponds  with  the  letter.  The  machinery 
by  which  this  is  effected  is  necessarily  complicated  and  it  falls 
not  within  the  province  of  this  work  to  g6  fu  ther  into  the 
explanation.  The  whole  process  is  described  in  Davis'  Book  of 
the  Telegraph,  to  which  this  volume  is  indebted  for  most  of  lhe 
particulars  which  have  been  giveii  in  relation  to  the  subject. 


ELECTRO  MAGNETIC    TELEGRAPH.  'j 

The  following  history  of  the  electric  telegraph  in  this  country  is  extracted 
fex'iu.  the  Portland  Advertiser,  and  deserves  a  place  in  this  connexion  : 

"  The  electric  telegraph,  being  used  solely  for  the  o^nveyance  of  news 
and  communications,  is  so  intimately  connected  with  posts  and  post-offices, 
*hat  a  brief  sketch  of  its  rapid  progress  in  the  United  States  is  here  given. 

"  It  is  to  American  ingenuity  that  we  owe  the  practical  application  of 
the  magnetic  telegraph  for  the  purpose  of  communication  between  distant 
points,  and  it  has  been  perfected  and  improved  mainly  by  American  scienc« 
and  skill.  While  the  honor  is  .due  to  Professor  Morse  for  the  practical 
application  and  successful  prosecution  of  the  telegraph,  it  is  mainly  owing 
to  the  researches  and  discoveries  of  Professor  Henry,  and  other  scientific 
Americans,  that  he  was  enabled  to  perfect  so  valuable  an  invention 

"The  first  attempt  which  was  made  to  render  electricity  available  for 
the  transmission  of  signals,  of  which  we  have  any  account,  was  that  of 
Lesage,  a  Frenchman,  in  1774.  From  that  time  to  the  present  there  have 
been  numerous  inventions  and  experiments  to  effect  this  object ;  and,  from 
1820  to  1850,  there  were  no  less  than  sixty-three  claimants  for  different 
varieties  of  telegraphs.  We  will  direct  attention  only  to  those  of  Morse, 
Bain  and  House,  they  being  the  only  kinds  use.d  in  this  country. 

"  During  the  summer  of  1832,  Professor  S.  F.  B.  Morse,  an  American,  con- 
ceived the  idea  of  an  elective  or  electro-magnetic  telegraph,  and,  after 
numerous  experiments,  announced  his  invention  to  the  public  in  April,  1837. 

"  On  the  iOth  of  March,  1837,  Hon.  Levi  Woodbury,  then  Secretary  of 
the  Treasury,  issued  a  circular  requesting  information  in  regard  to  the 
propriety  of  establishing  a  system  of  telegraphs  for  the  United  States,  to 
which  Professor  Morse  replied,  giving  an  account  of  his  invention,  its  pro- 
posed advantages  and  probable  expense.  At  that  time  *  he  presumed  five 
words  could  be  transmitted  in  a  minute.' 

"  In  1838,  the  American  Institute  reported  that  Morse  could  telegraph 
the  words  '  steamboat  Caroline  burnt '  in  six  minutes.  Now,  a  thousand 
such  words  are  telegraphed  in  two  minutes. 

"  In  1844,  Congress  built  an  experimental  line  from  Baltimore  to  Wash- 
ington, to  test  its  practical  operation.  That  line  was  soon  continued  on  to 
Philadelphia  and  New  York,  and  reached  Boston  the  following  year.  Two 
branches  diverge  from  this  line,  one  from  Philadelphia  to  St.  Louis,  1000 
miles,  the  other  from  New  York,  via  Buffalo,  to  Milwaukie,  1300  miles 
long.  One  also,  1400  miles  in  length,  goes  from  Buffalo  to  Lockport,  and 
from  thence  through  Canada  to  Halifax,  N.  S.,  whence  there  is  a  continuous 
line  through  Portland  to  Boston.  The  great  Southern  line,  from  Washing- 
ton to  New  Orleans,  is  1700  miles  long.  Another,  1200  miles,  running  to 
New  Orleans  from  Cleveland,  Ohio,  via  Cincinnati.  The  best  paying  line, 
it  is  said,  is  that  between  Washington  and  New  York,  which,  during  six 
months  of  last  year,  transmitted  154,514  messages,  valued  at  $68,499  ;  and 
the  receipts  for  the  year  ending  July,  1852,  were  $103,060.  The  average 
performance  of  the  Morse  instruments  is  from  8000  to  9000  letters  per 
hour.  The  cost  01  construction,  including  wire,  posts,  labor,  <fec.,  is  about 
$150  per  mile.  The  Bain  telegraph  extends  in  the  United  States  2012 
miles,  and  House's  2400  miles,  making  a  total,  with  Morse's,  of  89  lines, 
embracing  16,729  miles.  At  how  many  way  stations  the  magnetic  current 
is  arrested  and  messages  conveyed,  we  are  not  informed.  Thus,  in  less 
than  nine  years,  from  a  feeble  beginniMg,  under  the  fostering  aid  of  govern- 
ment, have  its  wires  apd  news  communications  spread  all  over  the  country. 

**  The*  astonishing  results  of  the  telegraph,  victorious  even  in  a  run 
against  time,  are  remarkable  in  the  United  States.  The  western  cities, 
having  a  difference  of  longitude  in  their  favor,  actually  receive  news  from 
New  York  sooner  by  the  clock  than  it  is  sent.  When  the  Atlantic  made 
Ler  first  return  voyage  from  Liverpool,  a  brief  account  of  the  newa  was 


NATUllAL    PHILOSOPHY. 

»elegra;-htd  to  New  Orleans  at  a  few  minutes  after  noon  (New  York  ci;i>-); 
and  reacheJ  its  destination  at  a  few  minutes  before  noon  (New  Crleans 
time),  and  was  published  in  the  evening  papers  of  both  cities  at  the  same 
hour.  This  is  now  a  daily  occurrence. 

"  Through  its  instrumentality  (we  mean  no  pun)  Webster's  deatb  was 
simultareuusly  made  known  throughout  the  length  and  breadth  of  our 
land,  and  the  next  morning  the  pulpits  from  Maine  to  New  Orleans  were 
echoing  in  e'u'ogies  to  his  greatness,  and  mourning  his  departure. 

"  The  great  extent  of  the  telegraph  business,  and  its  importance  to  the 
community,  is  shown  by  a  statement  of  the  amount  paid  for  despatches  by 
the  associated  press  of  New  York,  composed  of  the  seven  principal  morning 
papers,  —  the  Courier  and  Knijuirer,  Tribune.,  Herald,  Journal  of  Commerce, 
Sun,  Times  and  Express.  During  the  year  ending  November  1,  1852, 
these  papers  paid  nearly  $50,000  for  despatches,  and  about  $14,000 
*or  special  and  exclusive  messages,  not  included  in  the  expenses  of  the 
association. 

"The  difference  between  Morse's  and  House's  telegraph  is,  principally, 
that  the  first  traces  at  the  distant  end  what  is  marked  at  the  other  ;  while 
House's  does  not  trace  at  either  end,  but  makes  a  signal  of  a  letter  at  the 
distant  end  which  has  been  made  at  the  other,  and  thus,  by  new  machinery, 
and  a  new  power  of  air  and  axial  magnetism,  is  enabled  to  print  the  signal 
letter  at  the  last  end,  and  this  at  the  astonishing  rate  of  sixty  or  seventy 
strokes  or  brakes  in  a  second,  and  at  once  records  the  information,  by  its 
own  machinery,  in  printed  letters.  Morse's  is  less  complicated,  and  more 
easily  understood;  while  House's  is  very  difficult  to  be  comprehended  in  its 
operations  in  detail,  and  works  with  the  addition  of  two  more  powers, — 
one  air,  and  the  other  called  axial  magnetism.  One  is  a  tracing  or  writing 
telegraph,  the  other  a  signal  and  printing  telegraph. 

"  The  telegraphs  in  England  are  next  in  importance  and  extent  to  those 
in  this  country.  They  were  first  established  in  1845,  and  there  are  about 
4000  miles  of  wire  now  in  operation. 

"  The  charge  for  transmission  of  despatches  is  much  higher  than  in 
America,  one  penny  per  word  being  charged  for  the  first  fifty  miles,  and 
one  farthing  per  mile  for  any  distance  beyond  one  hundred  miles.  A 
message  of  twenty  words  can  be  sent  a  distance  of  500  miles  in  the  United 
States  for  one  dollar,  while  in  England  the  same  would  cost  seven  dollars  " 

1190.  THE  ELECTRICAL  FIRE  ALARM.  —  The  principle  of  the  electric 
telegraph  has  recently  been  applied  to  a  very  ingenious  pie?e  of 
mechanism,  by  which  an  alarm  of  fire  may  be  almost  instantly  com- 
municated to  every  part  of  a  large  city.  Wires,  extending  from 
the  towers  of  the  principal  public  buildings  in  which  large  bells 
are  suspended,  unite  at  a  central  point,  where  the  operator  is  in 
constant  attendance.  On  an  alarm  of  fire  in  any  locality,  the  watch 
or  police  of  the  district  goes  to  a  small  box,  kept  in  a  -onspicuous 
place,  which  he  opens,  and  makes  a  telegraphic  communication  to  the 
central  operator,  who,  immediately  recognizing  the  signal  and  the 
district  from  which  it  came,  gives  the  alarm,  by  making  each  bell 
in  connexion  with  the  telegraph  strike  the  number  corresponding 
with  the  district  in  which  the  alarm  commenced.  By  this  means 
the  alarm  is  communicated  simultaneously  to  all  parts  of  the  city. 
ThitJ  ingenious  application  of  scientific  principles  hap  been  in  suc- 
cessful operation  In  the  city  of  Boston  long  enough  to  prove  its 
gieat  value. 


ELECTKOTY PIS    PKOCESS.  331 

* 

1191.  THK  ATMOSPHERIC  TELEGRAPH. —  An  ingenious  appar 
atus,  called  "  The  Atmospheric  Telegraph"  has  recently  been 
constructed  by   Mr.  T.  S.  Richardson-,  of  Boston,   designed    to 
send  packages  through   continuous  tubes  by  means  of  atmos* 
pheric  pressure      An    air-tight   tube   being  laid  between   two 
places,  either  under  or  above  ground,  a  piston,  called  by  Mr.  R. 
a  plunger,  is  accurately  fitted   to    its  bore,  behind  which   the 
package  designed  to  be  sent  is  attached.     The  air  having  been 
exhausted  from  the  tube  by  engines  at  the  opposite  end,  the 
pressure  of  the  atmosphere  will  drive  the  piston,  or  plunger 
with  its  load,  forward  to  its  proposed  destination. 

This  ingenious  application  of  atmospheric  pressure  operates 
with  entire  success  in  the  model,  and  'has  been  also  successfully 
tested  in  tubes  that  have  been  laid  to  the  extent  of  a  mile.  Pa- 
tents have  been  secured  for  the  invention  in  England,  Franje, 
find  other  countries  of  Europe,  as  well  as  in  this  country ;  and 
a  company  is  now  forming  for  testing  the  principle  between  the 
cities  of  Boston  and  New  York.  The  air  is  to  be  exhausted 
from  the  tubes  by  means  of  steam-engines,  and  there  are  to  be 
intermediate  stations  between  those  two  cities. 

1192.  THE  ELECTROTYPE  PROCESS.  —  This  process,  known  by 
the  various  names  electrotype,  electro  plating  and  gilding,  gal- 
vanotype,  galvano-plastic,  electro-plastic  and  electro-metallurgy, 
is  a  process  by  which  a  coating  of  one  metal  is  made  to  adhere 
to  and  take  the  form  of  another  metal,  by  electrical  agency. 

1193.  It  is  a  process  purely  chemical  and  electrical,  and  the  con- 
sideration of  the  subject  pertains  more  properly  to  the  science  of 
Chemistry.     As  this  volume  has  not  professed  to  pursue  a  rigid 
classification,  it  may  not  be  amiss  to  give  this  brief  notice  of  the 
process. 

1194.  It  consists  in  subjecting  a  chemical  solution  of  one 
metal  to  electrical  action  with  another  metal.     A  solution  of  a 
salt  or  oxide,  having  a  metallic  base,  forms  part  of  the  electric 
circuit,  and,  by  the  electrical  action,  the  oxygen  or  acid  will  be 
drawn  to  the  positive  end  of  the  circuit,  while  the  pure  meta) 
vill  be  forced  to  the  negative  pole,  where  it  will  either  combine 

14* 


382  NATURAL   PHILOSOPHY. 

with  the  metal  or  adhere  to  it,  taking  its  exact  form.  The 
thickness  of  the  coating  of  the  pure  metal  will  depend  on  tiw 
length  of  time  that  the  body  to  be  coated  is  subjected  to  the 
combined  action,  chemical  and  electrical.  Hence  a  mere  film 
or  a  solid  crust  may  be  attached  to  any  conducting  substance. 

When  a  substance  not  in  itself  a  conductor  is  to  be  coated,  it 
must  first  be  made  a  conductor  by  covering  its  surface  with 
some  substance  which  will  impart  the  conducting  power.  This 
is  usually  effected  by  means  of  finely-powdered  black  lead. 

1195.  When  a  part  only  of  a  body  is  to  be  coated  by  tho 
electrotype  process,  the  parts  which  are  to  remain  uncoate^  must 
previously  be  protected  by  means  of  a  thin  covering  of  wax, 
tallow  or  some  other  non-conducting  substance. 

1196.     MAGNETO-ELECTRICITY.  —  Mag- 

What  is   Mag-  ,     A  .  .^  f      .        ,       , 

neto-electridty?    neto- electricity   treats  of    the  development 

of  electricity  by  magnetism. 

How  is   Mag-        1197.  Electric  currents  are  excited  in  a 

neto-electridty  conductor  of  electricity  by  magnetic  changes 
developed?  .  ,  .  ,  .  .  .  .  .  m,  ?L 

taking   place   in   its    vicinity.      Thus,    the 

movement  of  a  magnet  near  a  metallic  wire,  or  near  an  iron 
bar  enclosed  in  a  wire  coil,  occasions  currents  in  the  wire. 

1198.  When  an  armature,  or  any  piece  of  soft  iron,  ia 
brought  into  contact  with  one  or  both  of  the  poles  of  a  magnet, 
it  becomes  itself  magnetic  by  induction,  and  by  its  reaction 
adds  to  the  power  of  the  magnet :  on  the  contrary,  when 
removed  from  the  contact,  it  diminishes  the  power  of  the  mag- 
net, and  these  alternate  changes  in  its  magnetic  state  induce  a 
current  of  electricity. 

1199.  The  most  powerful  effects  are  obtained 

b?  cauaing  a  bar  of  soft  iron'  enclosed  in  a 
ffects  of  mag-  helix,  to  revolve  by  mechanical  means  near  the 
wto-ekctncity  p0ieg  Of  a  steej  magnet.  As  the  iron  approaches 

the  poles  in  its  revolution,  it  becomes  mag- 
netic; as  it  recedes  from  them,  its  magnetism  disappear* ;  *md 


MAGNETO- ELECTRICITY  . 

this  alternation  of  magnetic  states  causes  the  flow  of  a  current 
of  electricity,  which  may  be  directed  in  its  course  to  screw- 
cups,  from  which  it  may  be  received  by  means  of  wires  con- 
nected with  the  cups. 

1200.  TIIE   MAGNETO-ELECTRIC    MACHINE.- 
*£•       Fig.  181  represents  the   magneto-electric  ma- 
chine,  in  which   an  armature,  bent   twice   at 
right  angles,  is  made  to  revolve  rapidly  in  front  of  the  poles  of 
a  compound  steel  magnet  of  the  U  form.    The  U  magnet,  whose 


Explain 
181. 


aorth  pole  is  seen  at  N,  is  fixed  in  a  horizontal  position,  with  its 
poles  as  near  the  ends  of  the  armature  as  will  allow  the  latter 
to  rotate  without  coming  into  contact  with  them.  The  armature 
is  mounted  on  an  axis,  extending  from  the  pillar  P  to  a  small 
pillar  between  the  poles  of  the  magnet.  Each  of  its  legs  is 
enclosed  in  a  helix  of  fine  insulated  wire.  The  upper  part  of 
the  pillar  P  slides  over  the  lower  part,  and  can  be  fastened  in 
tiny  position  by  a  binding  screw.  In  this  way  the  band  con- 
necting the  two  wheels  may  be  tightened  at  pleasure,  by  in- 
creasing the  distance  between  them.  This  arrangement  aLso 
renders  the  machine  more  portable.  By  means  of  the  multiply- 
ing-wheel  W,  which  is  connected  by  a  band  with  a  small  wheel 
on  the  axis,  the  armature  is  made  to  revolve  rapidly,  so  that 
the  magnetism  induced  in  it  by  the  steel  magnet  is  alternately 


;*&!  NATUKAL    PHILOSOPHY. 

ilfi&troyed  and  renewed  in  a  reverse  direction  to  the  previous 
jne.  When  the  legs  of  the  armature  are  approaching  the  mag- 
net, the  one  opposite  the  north  pole  acquires  south  polarity,  and 
the  other  north  polarity.  The  magnetic  power  is  greatest  while 
the  armature  is  passing  in  front  of  the  poles.  It  gradually 
diminishes  as  the  armature  leaves  this  position,  arid  nearly  dis- 
appears when  it  stands  at  right  angles  with  the  magnet.  A* 
each  leg  of  the  armature  approaches  the  other  pole  of  the  U 
magnet,  by  the  continuance  of  the  motion  magnetism  is  again 
induced  in  it,  but  in  the  reverse  direction  to  the  previous  one. 
These  changes  in  the  magnetic  state  of  the  armature  excite 
electric  currents  in  the  surrounding  helices,  powerful  in  proper 
tion  to  the  rapidity  with  which  the  magnetic  changes  are  pro- 
duced. 

1201.  Shocks  may  thus  be  obtained  from  the  machine,  and,  if  the 
motion  is  very  rapid,  in  a  powerful  machine  the  torrent  of  shocks 
becomes  insupportable  —  the  muscles  of  the  hands  which  grasp  the 
handles  are  involuntarily  contracted,  so  that  it  is  impossible  to 
loosen  the  hold.  The  shocks,  however,  are  instantly  suspended  by 
bringing  the  metallic  handles  into  contact. 

1202.   THERMO-ELECTRICITY.  —  Thermo- 

What  is  Thar-      ,     ,    .  .  „  f     ,  -       .   . 

no-electricity?     electricity   expresses  a  form  of  electricity 

developed  by  the  agency  of  heat. 

1203.  In  the  year  1822,  Professor  Seebeck,  of  Lorlin,  dis- 
covered that  currents  of  electricity  might  be  produced  by  the 
partial  application  of  heat  to  a  circuit  composed  exclusively  of 
soJ.id  conductors.  The  electrical  current  thus  excited  has  been 
termed  Thermo-electric  (from  the  Greek  Thermos,  wbich  signi- 
fies heat),  to  distinguish  it  from  the  common  galvanic  current; 
which,  as  it  requires  the  intervention  of  a  fluid  element,  was 
denominated  a  Hydro-electric  current.  The  term  Stereo-electric 
current  has  also  been  applied  to  the  former,  in  order  to  mark 
its  being  produced  in  systems  formed  of  solid  bodies  alone.  It 
is  evident  that  if,  as  is  supposed  in  the  theory  of  Ampere,  mag- 
nets owe  their  peculiar  properties  to  the  continual  circulation 
of  electric  currents  in  their  minute  parts,  these  currents  will 
<\)iue  under  the  description  of  the  stereo-electric  currents 


TH UXMO ELECTRICITY.-  -A8TKOM  OMY.  rfSfi 

1204.  From  the  views  of  electricity  which  have  now  been 
given,  it  appears  that  there  are,  strictly  speaking,  three  state* 
of  electricity.     That  derived  from  the  common  electrical  ma- 
chine i&  in  the  highest  degree  of  tension,  and  accumulates  until 
it  is  able  to  force  its  way  through  the  air,  which  is  a  perfect 
non-conductor.     In  the  galvanic  apparatus  the  currents  have  a 
smaller  degree  of  tension;  because,  although  they  pass  freely 
through  the  metallic  elements,  they  meet  with  some  impedimenta 
in  traversing  the  fluid  conductor.     But  in  the  thermo-electric 
currents  the  tension  is  reduced  to  nothing  ;  because,  throughout 
the  whole  course  of  the  circuit,  no  impediment  exists  to  its  free 
and  uniform  circulation. 

1205.  If  the  junction  of  two  dissimilar  metals  be  heated,  an 
electrical  current  will  flow  from  the  one  to  the  other. 

1206.  Instead  of  two  different  metals,  one  metal  in  differen* 
Conditions  can  be  used  to  excite  the  current. 

1207.  Metals  differ  greatly  in  their  power  to  excite  a  cur- 
rent when  associated  in  thermo-electric  pairs.     A  current  may 
be  excited  with  two  wires  of  the  same  metal,  by  heating  the 
end  of  one,  and  bringing  it  into  contact  with  the  other.     Thi? 
experiment  is  mostr-successful  when  metals  are  used  that  have 
the  lowest  conducting  power  of  heat. 

1208.  Thermo-electric  batteries  have  been  constructed  with 
sufficient  power  to  give  shocks  and  sparks,  and  produce  various 
magnetic  phenomena,  indicative  of  great  magnetic  power ;  but 
the  limits  of  this  volume  will  not  allow  a  further  consideration 
of  the  subject. 

1209.  ASTRONOMY. —  Astronomy   treats 

What  is  Aslron-      r>  L,      -\  i     i     -i  •        ^i  i 

omyi  of  the  heavenly  bodies,  the  sun,  moon,  plan- 

ets, stars  and  comets,  and  of  the  earth  as  a 
member  of  the  solar  system. 

1210.  The  study  of  astronomy  necessarily  involves  an  acquaint- 
ance with  mathematics,  but  there  are  many  interesting  facts,  which 
have  Leen  fully  established  by  distinguished  astronomers,  whicb 
ought  to  be  familiar  to  those  who  huve  neither  the  opportunity  nor 


d8(.'  NATURAL    PHILOSOPHY. 

tJio  leisu/e  to  pursue  the  subject  by  the  aid  of  mathematical  light. 
To  such  the  following  brief  notice  of  the  subject  will  not  be  devoid 
Df  interest 

1211.  Some  of  the  most  distinguished  men  who 
Who  are  some  have  contributed  to  the  great  mass  of  facts  and 
tf  the  most  dis-  laws  which  make  up  the  science  of  Astronomy 
tinguished  As-  were  Hipparchus,  Ptolemy,  Pythagoras,  Coperni- 
tronomers  ?  cus,  Tycho  Brahe,  Galileo,  Kepler  and  Newton, 

The  present  century  has  added  to  this  list  many 
other*  whose  fame  will  descend  to  posterity  with  great  lustre. 

1212.  Hipparchus  is  usually  considered  the  father  of  Astronomy. 
He  was  born  at  Nicaea,  and*  died  about  a  hundred  and  twenty-five 
years  before  the  Christian  era.     He  divided  the  heavens  into  con- 
stellations, twelve  in  the  ecliptic,  twenty-one  in  the  northern,  and 
sixteen  in  the  southern  hemisphere,  and  gave  names  to  all  the  stars. 

He  discovered  the  difference  of  the  intervals  between  the  £utinn- 
nal  and  vernal  equinoxes,  and,  likewise,  by  viewing  a  tree  on  a 
plain,  and  noticing  its  apparent  position  from  different  places  of 
observation,  he  was  led  to  the  discovery  of  the  parallax  of  the  heav- 
enly bodies  ;  that  is,  the  difference  between  their  real  and  apparent 
position,  viewed  from  the  centre  and  from  the  surface  of  the  earth. 
He  determined  longitude  and  latitude,  fixing  the  first  degree  of  lon- 
gitude at  the  Canaries. 

1213.  Ptolemy  nourished  in  the  second  century  of  the  Christian 
era.     He  was  a  native  of  Alexandria,  or  Pelusium.     In  his  system 
he  placed  the  earth  in  the  centre  of  the  universe,  —  a  doctrine  univer- 
sally adopted  and  believed  until  the  sixteenth  century,  when  it  was 
confuted  and  rejected  by  Copernicus.     Ptciemy  gave  an  account  of 
the  fixed  stars,  and  computed  the  latitude  and  longitude  of  one  thou- 
sand and  twenty-two  of  them. 

1214.  Pythagoras  was  born  at  Samos,  and  his  death  is  supposed 
to  have  taken  place  about  five  hundred  years  before  the  Christian 
era.     He  supposed  the  sun  to  be  the  centre  of  the  universe,  and 
that  the  planets  revolved  around  him  in  elliptical  orbits.     This  doc- 
trine, however,  was  deemed  absurd  until  it  was  established  by  Co- 
pernicus in  the  sixteenth  century. 

1215.  Tycho  Brahe,  a  Danish  astronomer,  flourished  about  the 
middle  of  the  sixteenth  century.     His  astronomical  system  was  sin- 
gular and  absurd,  but  the  science  is  indebted  to  him-  for  a  more  cor- 
rect catalogue  of  the  fixed  stars,  and  for  discoveries  respecting  the 
motions  of  the  moon  and  the  comets,  the  refraction  of  the  rays  of 
light,  and  for  many  othei  important  improvements.     To  him,  also, 
was  Kepler  indebted  for  the  principal  facts  which  were  the  basis  of 
nis  astronomical  labors. 

1216.  Copernicus  v~<is  born  in  Prussia,  in  the  latter  part  of  the 
fifteenth   century.     Ho   revived   the   system   of  Pythagoras,  which 
placed  the  sun  in  the  centre  of  the  system.     He  taught  the  true 
doctrine  that  the  aarent  motion  of  the  heavenl    bodies  is  caused 


apparent  motion  of  the  heavenly 
n  of  the  earth.     But,  for 
publication  of  his  system,  lie  gained  but 


by  the  real  motion  of  the  earth.     But,  for  nearly  a  century  after 
the  publication  of  his  sstem,  lie  gained  but  few  followers. 


ASTRONOMY.  331 

1217  Galileo,  a  native  of  Pisa,  flourished  in  the  latter  part  of 
the  pixteenth  century.  By  his  observation  of  the  planets  Venus  and 
j  upiter,  he  gained  a  decisive  victory  for  the  Copermcan  system.  He 
•rfjis  persecuted  and  imprisoned  by  the  inquisition  for  holding  what 
was  thought,  in  that  age  of  ignorance  and  superstition,  to  be  heret- 
ical opinions,  and  compelled  on  his  knees  to  abjure  the  truths 
whi  :h  he  had  discovered,  and  which  he  had  too  much  sense  to  dis- 
believe. Notice  has  already  been  taken  of  this  distinguished  phi- 
losopher in  connexion  with  the  laws  of  falling  bodies  (see  page  52), 
for  tii-D  discovery  of  which  the  world  is  indebted  to  him. 

1213.  Kepler,  who,  from  his  great  discoveries,  is  called  the  legis- 
ator  of  the  heavens,  was  a  native  of  Wirtemberg,  in  1571.  Availing 
himself  of  the  observations  of  Tycho  Brahe,  he  discovered  three 
great  laws,  known  as  Kepler's  laws  of  the  planetary  motions,  and  on 
them  were  founded  the  discoveries  of  Newton,  as  well  as  the  whole 
modern  theory  of  the  planets. 

Kepler's  laws  could  not  have  been  discovered  but  for  the  observa- 
tions of  Tycho  Brahe  (as  Kepler  was  not  himself  an  observer),  and 
no  further  discoveries  could  have  been  made  than  Kepler  made  but 
for  the  telescope  of  Galileo.  It  has  elsewhere  been  stated  that 
Galileo  was  indebted  to  Jansen,  of  Holland,  for  the  idea  of  the 
telescope.  But.  since  the  days  of  Galileo,  the  telescope  has  been 
most  wonderfully  improved,  and  invested  with  almost  inconceivable 
powers.  Herschel  computed  that  the  power  of  his  telescope  was  so 
great  as  to  penetrate  a  space  through  which  light  (moving  with  the 
prodigious  velocity  of  200,000  miles  in  a  second  of  time)  would 
require  350,000  years  to  reach  us.  But  the  great  telescope  of  Lord 
Rosse  would  probably  reach  an  object  ten  times  more  remote. 

1219.  Sir  Isaac  Newton,  who  has  been  called  the  Creator  of 
Natural  Philosophy,  was  born  in  Lincolnshire,  England,  in  1642. 
His  discovery  of  the  universal  law  of  gravitation,  and  many  other 
valuable  and  important  contributions  which  he  made  to  science., 
place  him  among  the  foremost  of  those  to  whom  the  world  is  in- 
debted for  an  insight  into  the  magnificent  displays  of  the  material 
world. 

1220.    According   to   the   system   of  As- 

Crtve  an  ac- 
count of  the       tronomy  which    is    now   universally   adopted, 
wlar  system  as   ^     sun  >     ^  tre  of  &  system  Of  heavenly 

now  adopted.  J 

bodies,   called  planets,  which  revolve  around 

him  as  a  centre. 

Secondly.    The  earth  is  one  of  these  planets. 

Thirdly.  Some  of  these  planets  are  attended  by  satel- 
lites or  moons,  which  revolve  around  their  respective 
planets,  and  with  them  around  the  sun. 


338  KATUBAL   PHILOSOPHY. 

Fourthly.  The  size,  distance  and  rapidity  of  motion  of 
each  of  these  planets  is  known  to  be  different. 

Fifthly.  The  stars  are  all  of  them  suns,  with  systems 
of  their  own,  and  probably  many,  if  not  all  of  them, 
having  planets,  with  their  moons  revolving  around  them 
as  centres. 

Sixthly.  There  is  a  central  point  of  the  universe,  around 
which  all  systems  revolve. 

Whatismeant  1221«  °F  THE  S°LAR  SYSTEM.-By  the 
by  the  Solar  Solar  System  is  meant  the  sun  and  all  the 
System?  heavenly  bodies  which  revolve  around  it. 

These  are  the  planets  with  their  satellites  or  moons,  our 
earth  with  its  moon,  together  with  an  unknown  number 
of  comets. 

What  are  1222-  OF  THE  PRIMARY  PLANETS.— Those 

Primary  bodies  which  revolve  around  the  sun,  with- 
Planete?  out  revolving,  at  the  same  time,  around 
some  other  central  body,  are  called  Primary  Planets. 

1223.  For  many  years  the  planets  were  con- 
Gwethenames  u      •    •  i 

of  the  eight       sidered  to  be  six  in  number  only,  and  they  were 

primary  all,  except  our  earih,  named  after  the  gods  Oi 

planets.  heathen    mythology,— Mercury,    Venus,    Earth, 

Mars,  Jupiter,  and  Saturn.  In  the  year  1781,  Sir  William 
Herschel  discovered  another,  to  which  the  name  of  Uranus  has 
been  given;  an<^  in  the  year  1846  an  eighth  was  discovered,  to 
which  the  nan  e  of  Le  Verrier  was  at  first  given,  from  a  dis- 
tinguished Fr-^i  ch  astronomei,  by  moans  of  whom  it  was  pointed 
out.  It  is  now  known  by  the  name  of  Neptune. 

How  many  1224.  Besides  these  primary  planets,  it  was 

minor pri-        discovered,  between   the  years  1800  and  1807, 

mary  planets  ,  / 

have  been  dis-  that  between  Mars  and  Jupiter  there  were  TOUT 

covered?  smaller  planets,  of  such  diminutive  size,  compared 

with  the  others,  that  they  were  called  Asteroids.  Since  the  year 
1845  one  hundred  and  nine  more  have  been  discovered,  so  that 


ASTRONOMY. 


339 


there  are  now  known  to  be  no  fewer  than  one  hundred  and 
thirteen  asteroids,  or  minor  planets,  between  the  orbits  of  Mars 
and  Jupiter. 

1225.  THE  MINOR  PLANETS. — The  following  is  a  catalogue 
of  the  minor  planets  at  present  known,  arranged  in  the  order 
of  their  discovery,  together  with  the  other  known  planets  of  out 
solar  system  : 


Nime  and  Number  by  which 
the  Minor  Planets  are  known. 

Date  of  Discovery. 

Names  of  Discoverers. 

Sira. 
MERCUKY 
VENUS. 
THE  EAETU. 
MARS. 
1    Ceres         .  .  . 

1801..  Jan.  1    . 

Piazzi,  of  Sicily. 

2.  Pallas  

1  802..  March  28... 

Gibers,  of  liromoii. 

8.  Juno  

1804.  .Sept.  1  

Harding. 

4   Vesta 

1807..  March  29.. 

Olbers. 

5.  Astrea  
6.  Hebe  

1845..  Dec.  8  
1847.  .July  1  

1  1  en  eke,  of  Germany. 
Ilencke. 

7.  Iris  
8.  Flora  

1847..  August  13... 
1847.  .Got  18  

Hind,  of  London. 
Hind. 

9.  Metis  

1848..  April  26  

Graham,  of  Ireland 

10    Hy'eia 

1849    April  12 

11.  Parthenope 

1850..  May  11    ... 

De  Gasparis. 

12    Clio 

1850    Sept  18... 

Hind 

13.  Egeria  

1850..  Nov.  2  

De  Gasparis. 

14.  Irene       

1851..  May  19... 

Hind. 

15.  Eunomia  

1851..  July  29 

De  Gasparis. 

16    Psyche 

1852     March  17 

De  Gusparis. 

17.  Thetis  

1852..  April  17  

Luther,  of  Germany. 

18   Melpomene 

1852    June  25 

Hind 

19.  Fortuna  

20.  Massilia  
21    Lutetia 

1852..  August  22... 
1852..  Sept.  19  
1852     Nov  15 

Hind. 
Do  Gasparis. 

22.  Calliope  .. 

lS52..Nov  16  . 

Hind. 

23.  Thalia  *.... 

24.  Themis 

1852..  Deo.  15  
1853     April  5 

Hind. 
De  Gasparis. 

25    Phoca-a 

1S53    April  6 

26.  Proserpina  

1853    May  5 

Luther. 

27.  Euterpe  

28.  Bellona  

lS53..Nov.  8  
1  854..  March  1  

Hind 
Luther. 

29    Aiiiphitrite 

1S54    March  1 

80.  Urauia  
81.  Euphrosyne  

32    Pomona 

1854..  July  22  
1854..  Sept  1..:... 
1854    Get  98 

Hind. 
Ferguson,  of  Washington. 
Goldschmidt 

33.  Polhymnia  

1854  .Get  28  ... 

Chacornac. 

84  
85  
JUPITER. 
SATURN. 
URANUS  

1855..  April  14  
1855..  April  27. 

1131 

Chacornac. 
Sir  William  Ilerschel. 

NKPTUNE  1  

1846..  Sept  23...  -j 

Dr.  Galle,  of  Berlin,  by  d'reo 
tion  of  Le  Verrier,  of  Paris. 

The  112th  asteroid  was  discovered  in  September,  1870.  The  113th,  in 
March  ;  the  114th,  in  July,  1871.  The  honor  of  many  late  discoveries  of 
these  bodies  rests  with  Frof.  Watson  of  this  country. 


NATDKAL     1»H1J/)S<  >l'H  V. 


What  is  the  1%2G.  The  name  planet  properly  means 
tiifftrence  be-  a  wandering  star,  and  was  given  to  this 
tween  a  planet  class  of  the  heavenly  bodies  because  they 
and  a  star?  ,,  .  ,  .,  t,  .  ,. 

are  constantly  moving,  while  those  bodies 

wfcich  are  called  fixed  stars  preserve  their  relative  posi- 
tions. The  planets  may  likewise  be  distinguished  from 
the  fixed  stars  by  the  eye  by  their  steady  light,  while 
ilic  fixed  stars,  on  the  contrary,  appear  to  twinkle. 

1227.  The  sun,  the  moon,  the  planets,  and  the  fixed  stars, 
which  appear  to  us  so  small,  are  supposed  to  be  large  worlds, 
of  ;  arious  sizes,  and  at  different  but  immense  distances  from  us. 
The  reason  that  they  appear  to  us  so  small  is,  that  on  account  of 
their  immense  distances  they  are  seen  under  a  small  angle  of  vision. 

Whatuniver-  1228.  It  has  been  stated,  in  the  early  pages 
sallow  keeps  of  this  book,  that  every  portion  of  matter  is  at- 

^ra«^ftracted  by  «™y  other  p°rtion'  and  *«  tha 

bodies  in  their  force  of  the  attraction  depends  upon  the  quantity 
places?  Oy  maf(er  and  the  distance.  As  attraction  is 

mutual,  we  find  that  all  of  the  heavenly  bodies  attract  the 
earth,  and  the  earthr  likewise  attracts  all  of  the  heavenly  bodies. 
It  hr.s  been  proved  that  a  body  when  actuated  by  several  forces 
will  be  influenced  by  each  one,  and  will  move  in  a  direction 
between  them.  It  is  so  with  the  heavenly  bodies  ;  each  one  of 
them  is  attracted  by  every  other  one  ;  and  these  attractions  are 
BO  nicely  balanced  by  creative  wisdom,  that,  instead  of  rushing 
together  in  one  mass,  they  are  caused  to  move  in  regular  paths 
Ccalled  orbits)  around  a  central  body,  which,  being  attracted  in 
different  directions  by  the  bodies  which  revolve  around  it,  will 
its^f  revolve  around  the  centre  of  gravity  of  the  system.  Thus, 
the  sun  is  the  centre  of  what  is  called  the  solar  system,  and  the 
planets  revolve  around  it  in  different  times,  at  different  dis- 
tances, and  with  different  velocities. 

1229.  The  paths  or  courses  in  which  the 
Planets  move  arounJ  the  sun  are  called 
their  orbits. 


ASTKONOMY. 


All  of  the  heavenly  bodies  move  in  conic  sections,*  namely,  itf 
circle,  the  ellipse,  tins  parabola  and  the  hyperbola. 

What  is  meant  1230.  In  obedience  to  the  universal  law  of 
by  a  year .  gravitation,  the  planets  revolve  around  the 
sun  as  the  centre  of  their  system ;  and  the  time  that  each 
one  takes  to  perform  an  entire  revolution  is  called  its  year 
Thus,  the  planet  Mercury  revolves  around  the  sun  in  87 
of  our  days ;  hence  a  year  on  that  planet  is  equal  to  87 
days.  The  planet  Venus  revolves  around  the  sun  in  224 
days ;  that  is,  therefore,  the  length  of  the  year  of  that 
planet.  Our  earth  revolves  around  the  sun  in  about  ^65 
days  and  6  hours.  Our  year,  therefore,  is  of  that  length. 

1231.  The  length  of  time  that  each  planet  take.*  in  perform- 
ing its  revolution  around  the  sun,  or,  in  other  words,  the  length 
of  the  year  on  ea«>a  planet,  is  as  follows.  (The  fractional 
parts  of  the  day  are  omitted.}  In  the  same  connexion  will  also 
be  found  the  mean  distance  of  each  planet  from  the  sun.  -ind 
the  time  of  revolution  arcur.d  its  axis ,  or,  in  other  words  the 
length  of  the  day  on  each. 


Length  of  the  Year  in 
Days. 

Mean  distance  from 
the  Sun 
in   millions   of  Miles. 

Length  of  lb« 
Day   in    Hours 
and  UfentM. 

8T 

86* 

24    & 

224 

SQl 

23  <\ 

EAKTH       

365 

95 

24  00 

MARS                                  

686 

145 

24  89' 

1    Ceres                               .   .. 

1.6^0 

2.  Pallas  

1.6S8 

8   Juno          

\  .5H2 

4  Vosta                               

1,825 

6.  Hebe        

1    Iris 

8    Flora 

9    Metis 

About  266 

10   Hygeia       

Between  1,400 

11    Parthenope 

and  2,100 

12    Clio 

13.  Egeria  

14.  Irene      

15.  Eunomla  _ 

16   Psyche 

1886 

*  Conic  sections  are  curvilinear  figures,  so  called  because  they  can  all  be 
formed  by  cutting  a  cone  in  certain  directions.  If  a  cone  be  cut  perpendicu- 
lar to  its  axis,  the  surface  cut  will  be  a  circle.  If  cut  oblique  to  the  axis, 
the  surface  cut  will  be  an  ellipse.  If  cut  parallel  to  the  slope  of  the  cone, 
the  section  will  be  a  parabola.  If  cut  parallel  to  the  axis,  the  section  will  ba 
' 


an 


NATUKAL    PHILOSOPHY. 


Length  of  the  Year 
in  Days. 

Mean  Distance  from 
the  Sun  in  millions 
of  Miles. 

Lentil  of  iba 
Dav  in   HOM* 
aud  A!i,ini«» 

17.  Thetis 

1,430             "1 

18.  Mel(H.nione  

1,269 

19.  Fortima  

1,896 

20.  Ma-^ilia        .     . 

1,859 

21.  Lutetia 

1,887 

About  266 

22    (;allinpe 

1  S15 

23.  Thalia  

1,571 

24.  Themis  

2,037 

25    Pliocaea        

26.  Proserpina  

27.  Euterpe  

28.  Bellona      

29.  Arnnliitrite 

80.  Urania  

81.  Enphrasyne  

82.  Pomona 

83.  Polhymnih. 

85.  f  - 
JUPITER 

4882 

494 

9  56' 

SATURN  

10.759 

906 

10  1« 

URANUS  

80  (JS6 

1  S24 

NEPTUNE 

60  126 

2856 

Give  an  ac- 
count of 
Bode's  law. 


The  sun  turns  on  its  axis  in  about  26  days  and  10  hours. 

1232.  There  is  a  very  remarkable  law,  dis- 
covered by  Professor  Bode,  founded,  it  is  true, 
on  no  known  mathematical  principle,  but  which 
has  been  found  to  accord  so  exactly  with  other  calculations, 
that  it  is  recognized  as  Bode's  law  for  estimating  the  distance 
of  the  planets  from  the  sun.     Thus  : 
Write  the  arithmetical  progression, 

0,  3,  6,  12,  24,  48,  96,  192,  384. 
To  each  of  the  series  add  4,  and  we  have  the  sums, 

4,  7,  10,  16,  28,  52,  100,  196,  388, 

which  will  represent  very  nearly  the  comparative  distance  of 
each  planet.  Now,  the  distance  of  the  earth  from  the  sun  is 
92  millions  of  miles,  and  as  that  distance  is  represented  in  the 
progression  by  10,  it  follows  that  the  distance  of  Mercury  is  7*6 
of  92  millions,  of  Venus  ^g,  &c. 

Wh  tied  t  1233.  It  is  to  be  observed,  however,  that  before 

the  discovery     tne  discovery  of  the  minor  planets,  there  was  a 
of  the  minor     very  remarkable    interval    between    the    planet.1 
Jupiterj  and  that   Bode>s  law/ 


planets  f 


to  accord  with  the  ^distance  of  all  the  other  planets, 


ASTRONOMY.  343 

appeared  here  to  fail  in  its  application.  Kepler  had  suspected 
that  an  undiscovered  planet  existed  in  the  interval  ;  but  it  was 
not  certainly  known  until  a  number  of  distinguished  observers 
assembled  at  Lilienthal,  in  Saxony,  in  1800,  who  resolved  to 
direct  their  observations  especially  to  that  part  of  the  heavens 
where  the  unknown  planet  was  supposed  to  be.  The  result  of 
the  labors  of  these  observers,  and  others  who  have  followed 
them,  has  been  the  discovery  of  the  one  hundred  and  thirteen 
minor  planets,  all  situated  between  the  planets  Mars  and  Jupiter. 
What  opinion  ^ut  tnese  minor  planets  are  so  small,  and  their 
has  been  formed  paths  or  orbits  vary  so  little,  that  it  has  been 
conJectured  that  they  originally  formed  one 
large  and  resplendent  orb,  which,  by  the  opera- 
tion of  some  unknown  cause,  has  exploded  and  formed  the 
minor  planets  that  revolve  in  orbits  very  near  that  of  the  original 
planet. 

1234.  Of  these  thirty-five  small  bodies,  which  are  quite  invisible 
without  the  aid  of  a  good  telescope,  ten  were  discovered  by  Mr. 
Hind,  of  Mr.  Bishop's  private  observatory,  Regent's  Park,  London; 
seven  by  De  Gasparis,  of  Naples  ;  three  by  Chacornac,  at  Marseilles  ; 
three  by  Luther,  at  Bilk,  Germany  ;  two  by  f)lbers,  of  Bremen  ;  two 
by  Hencke,  of  Dries^en,  Germany  ;  two  by  Goldschmidt,  at  Paris  ; 
and  one  each  by  Piazzi,  of  Palermo;  Harding,  of  Lilienthal,  Ger- 
many; Graham,  at  Mr.  Cooper's  private  observatory,  Markree 
Castle,  Ireland  ;  Marth,  of  London  ;  and  Ferguson,  of  Washington. 

1235.  The  paths  or  orbits  of  the  planets 
What  ts  the  •       i          -.  iv   ^     i 

shape  of  the     *re   no*    exactly   circular,   but   elliptical. 

orbits  of  the  They  are,  therefore,  sometimes  nearer  to 
p  a  the  sun  than  at  others.  The  mean  distance 

is  the  medium  between  their  greatest  and  least  distance. 
Those  planets  which  are  nearer  to  the  sun  than  the 
earth  are  called  inferior  planets,  because  their  orbits  are 
within  that  of  the  earth  ;  and  those  which  are  further 
from  the  sun  are  called  superior  planets,  because  their 
orbits  are  outside  that  of  the  earth. 


Qwe  the  rda-  1236    The  relatiye  size  of  the   srm    the 

live  size  of  the  •,  -, 

tun,  moon,  moon  and  the  larger  planets,  as  expressed  by 

and  primary  the  length  of  their  diameters,  is  as  follows  : 
ylo/uets. 


344 


NATURAL   PHILOSOPHY. 


Sun  .  . 
Moon  .  . 
Mercury  . 
Venus .  . 
Earth  . 


.  852,000 
.  2,153 
.  2,962 
.  7,510 
7,912 


Mars  .  . 
Jupiter  . 
Saturn  . 
Uranus  . 
Neptune  . 


4,920 
85,390 
71,904 
33,024 

36,620 


How  large  are 
the  minor 
planets  ? 


1237.  The  size  of  the  minor  planets  has  been 
so  variously  estimated,  that  little  reliance  can  be 
placed  on  the  calculations.  Some  astronomers 
estimate  them  as  a  little  over  1000  miles,  while  others  place 
them  much  below  that  standard.  Vesta  has  been  described  as 
presenting  a  pure  white  light ;  Juno,  of  a  reddish  tinge,  and 
with  a  cloudy  atmosphere  ;  Pallas  is  also  stated  as  having  a 
dense,  cloudy  atmosphere ;  and  Ceres,  as  of  a  ruddy  color. 
These  four  undergo  various  changes  in  appearance,  and  but 
little  is  known  of  any  of  them,  except  their  distance  arid  time 
of  revolution. 

Explain          1238.  Fig.  182  is  a   representation  of  the   com 

/<%.  182.   parative  size  of  the  larger  planets. 

• 

Fig.  182. 


.Sir  J.  F.  W.  Herschel  gives  the  following  illustration  of  the  com- 
parative size  and  distance  of  the  bodies  of  the  solar  system.  "  On 
a  well-levelled  field  place,  a  globe  two  feet  in  diameter,  to  represent 
the  Sun  ;  Mercury  will  be  represented  by  a  grain  of  nmstafd-seed.. 
on  the  circumference  of  a  circle  164  feet  in  diameter  for  its  orbit  • 
Venus,  a  pea,  on  a  circle  284  feet  in  diameter;  the  Earth,  also  u 
|V:a,  on  a  eirvle  of  4ijO  feel  ,  Mars,  a.  rather  largo  pin's  head,  on  a 


ASTKO.NUMY.  <H5 

drr'e  of  654  feet;  Juno,  Ceres,  Vesta,  and  PaLas,  grains  of  sand, 
in  orbits  of  from  1,000  to  1,200  feet;  Jnpiter,  a  moderate-sized 
orange,  in  a  circle  nearly  half  a  mile  in  diameter ;  Saturn,  a  small 
orange,  on  a  circle  of  four-fifths  of  a  mile;  Uranus,  a  full-sized 
cherry,  or  small  plum,  upon  the  circumference  of  a  circle  more  than 
a  mile  and  a  half;  and  Neptune,  a  good-sized  plum,  on  a  circle 
about  two  miles  and  a  half  in  diameter. 

"  To  imitate  the  motions  of  the  planets  in  the  above-mentioned 
orbits,  Mercury  must  describe  its  own  diameter  in  41  seconds; 
Venus,  in  4  minutes  and  14  seconds;  the  Earth,  in  7  minutes; 
Mars,  in  4  minutes  and  48  seconds ;  Jupiter,  in  2  hours,  66  minutes ; 
Saturn,  in  3  hours,  13  minutes;  Uranus,  in  12  hours,  16  minutes; 
and  Neptune,  in  3  hours,  30  minutes." 

1239.  The  Ecliptic  is  the  apparent  path 
What  is  the        „    ,  ,,  ,        ,1       /.  Ti  ,1 
Ecliptic,  and  °*  the  sun>  or  tne  rca*  Patn  °*  the  earth. 
why  is  it  so         It  is  called  the  ecliptic,  because  every 

eclipse,  whether  of  the  sun  or  the  moon, 
must  be  in  or  near  it. 

1240.  The  Zodiac  is  a  space  or  belt,  six- 
Zoctiac?          ^een  degrees  broad,  eight  degrees  eacli  side 

of  the  ecliptic. 

It  is  called  the  zodiac  from  a  Greek  word,  which  sig- 
nifies an  animal,  because  all  the  stars  in  the  twelve 
parts  into  which  'the  ancients  divided  it  were  formed 
into  constellations,  and  most  of  the  twelve  constellations 
were  called  after  some  animal. 

1241.  Sir  J.  F.  W.  Herso.hel,  in  his  excellent  treatise  on  As- 
tronomy, says :  "  Uncouth  figures,  and  outlines  of  men  and  iiion- 
sters  are  usually  scribbled  over  celestial  globes  and  maps,  and 
serve,  in  a  rude  and  barbarous  way,  to  enable  us  to  talk  of  groups 
of  stars,  or  districts  in  the  heavens,  by  names  which,  though  alv-uird 
or  puerile  in  their  origin,  have  obtained  a  currency  from  which  it 
would  be  difficult  to  dislodge  them.  In  so  far  as  they  have  really 
(as  some  have)  any  slight  resemblance  to  the  figures  called  up  in 
imagination  by  a  view  of  the  more  splendid  '  constellations,  they 
have  a  certain  convenience;  but  as  they  are  otherwise  entirely  ar- 
bitrary, and  correspond  to  no  natural  subdivisions  or  groupings 
of  the  stars,  astronomers  treat  them  lightly,  or  altogether'disregard 
them,  except  for  briefly  naming  remarkable  stars,  as  '•Alpha,  Le&nis? 
'•Beta  Scorpii,'  &c.,  by  letters  of  the  Greek  alphabet  attached  to  them. 

"  This  disregard  is  neither  supercilious  nor  causeless.  The  con- 
stellations seem  to  have  been  almost  purpose  b  nained  and  delineated 


546 


NATURAL    PHILOSOPHY. 


to  cause  as  much  confusion  and  inconvenience  as  possible.  Ir* 
numerable  snaked  twine  through  long  and  contorted  areas  of  tho 
heavens,  where  no  memory  can  follow  them  ;  bears,  lions,  and 
fishes,  large  and  small,  northern  and  southern,  confuse  all  nomen- 
clature, &c.  A  better  system  of  constellations  might  have  been  a 
material  help  as  an  artificial  memory." 

What  arc  the  1242.  The  zodiac  is  divided  into  twelve 
sign*  of  i  he  signs,  each  sign  containing  thirty  degrees  of 
many  degrees  in  tuc  great  celestial  circle.  The  names  of  these 
wch  ?  signs  are  sometimes  given  in  Latin,  and 

sometimes  in  English.     They  are  as  follows  : 


Latin.  English. 

•I)  Aries,     The  Ram. 

2)  Taurus,  The  Bull. 

3)  Gemini,  The  Twins. 
:  I)  Cancer,  The  Crab. 
|5)  Leo,        The  Lion. 
'())  Virgo,    The  Virgin. 


Latin.  English. 

(7)  Libra,  The  Balance. 

(8)  Scorpio,  The  Scorpion. 

(9)  Sagittarius,  ^  The  Archer. 

(10)  Capricornus,  The  Goat. 

(11)  Aquarius,       The  Water-bearer. 

(12)  Pisces,  The  Fishes. 


1243.  The  signs  of  the  zodiac  and  the  various  bodies  of  the 
solar  system  arc  often  represented,  in  almanacs  and  astronomical 
works,  by  signs  or  characters. 

In  the  following  list  the  characters  of  the  planets,  &c..  are 
I  ('presented. 

©  The  Sun.  ®  The  Earth.  £   Ceres. 

<£  The  Moon.  £    Mara.  $    Pallas. 

£    Mercury.  g   Vesta.  %  Jupiter. 

9    Venus.  cj>    Juno.  \i    Saturn. 

$  Uranus. 

The  following  characters  represent  the  signs  of  the  Zodiac* 

£\,  Leo.  $  Sagittarius. 

TTJJ  Virgo.  >J  Capricornus. 

£±  Libra.  zz  Aquarius. 

TH.  Scorpio.  X  Pisces. 

Prom  an  inspection  of  Fig.  183  it  appears  that  when  the  earth 


°K°  Aries. 

y  Taurus. 

U  Gemins. 

IB  Cancer. 


ASTRONOMY.  ,54'7 

a?  rieeu  from  the  SUD,  is  in  any  particular  constellation,  the  sun 
as  viewed  from  the  earth,  will  appear  in  the  opposite  one. 

Hare  the  signs  1244.  The  constellations  of  the  zodiac  do  not 

of  the  zodiac  now  retajn  ^c{r  original  names.     Each  con- 

3/ioayt   remain-  ;-«',.        .      ,         ^/T  i 

sd  the  same,  stellation  is  about  30  degrees  eastward  of  the 

and  why?  sign  of  the  same  name.     For  example,  the  con- 

stellation Aries  is  30  degrees  eastward  of  the  sign  Aries,  and  the 
constellation  Taurus  30  degrees  eastward  of  the  sign  Taurus,  and 
so  on.  Thus  the  sign  Aries  lies  in  the  constellation  Pisces  ;  the 
sign  Taurus,  in  the  constellation  Aries  ;  the  sign  Gemini,  in  the 
constellation  Taurus,  and  so  on.  Hence  the  importance  of  dis- 
tinguishing between  the  signs  of  the  zodiac  and  the  constellations 
of  the  zodiac.  The  cause  of  the  difference  is  the  precession  of 
the  equinoxes,  a  phenomenon  which  will  be  explained  in  its  propei 
connexion. 


t  1245.    The   orbits  of  the  other  planets 

orbits  of  the  •     T  '    i    i      ,1     ,       c     ^  ^ 

planets  situated    are   inclined  to  that  of  the  earth  ;    or,  m 

with  respect  to  other  words,  they  are  not  in  the  same 
that  of  the 

earth?  Plane' 

Explain  Fig.  183  represents  an  oblique  view  of  the  plane 

Fig.  183.  of  the  ecliptic,  the  orbits  of  all  the  primary  planets, 
and  of  the  comet  of  1680.  That  part  of  each  orbit  which  is 
above  the  plane  is  shown  by  a  white  line  ;  that  which  is  below 
it,  by  a  dark  line.  That  part  of  the  orbit  of  each  planet  where 
it  crosses  the  ecliptic,  or,  in  other  words,  where  the  white  and 
dark  lines  in  the  figure  meet,  is  called  the  node  of  the  planet, 
from  the  Latin  nodus,  a  knot  or  tie. 

Ejcplain  1246.  Fig.  184  represents  a  section  of  the  plane 

Fig.  184.  Of  the  ecliptic,  showing  the  inclination  of  the  orbits 
of  the  planets.  As  the  zodiac  extends  only  eight  degrees  on 
each  side  of  the  ecliptic,  it  appears  from  the  figure  that  the 
orbits  of  some  of  the  planets  arc  wholly  in  the  zodiac,  while 
those  of  others  rise  above  and  descend  below  it.  Thus  the 
orbits  of  Juno,  Ceres  and  Pallas,  rise  above,  while  those  of  all 
the  other  planets  are  confined  to  the  zodiac. 
15 


848 


NATURAL  PIJILOSOPHY. 


ASTRONOMY. 


When  is  a  1247.   When  a  planet  or  heavenly  body 

heavenly  body        .     .     ,,    ,  ,,  .^         ,.,      ,  .  , 

w  m  tnat  Part  <»  lts  orDlt  which  appears  to 


constellation  f       fa  near  aily  particular  constellation,  it  is  said 
to  be  in  that  constellation. 

This,  in  Fig.  147,  the  comet  of  1680  appears  to  approach  the 
aun  from  the  constellation  Leo.  . 

mat  is  meant         1248-  The    perihelion*   and    aphelion* 
by  the  perihelion    of  a  heavenly  body  express  its  situation  with 

£££?£  re§ard  to  the  sun-  When  a  **%  is  nearest 

gee,  of  a  heavenly  to  the  sun,  it  is  said  to  be  in  its  perihelion- 
When  furthest  from  .  the  sun,  it  is  said  to 
be  in  its  aphelion. 

1249.  The  earth  is  three  millions  of  miles  nearer  to  the 
flun  in  its  perihelion  than  in  its  aphelion. 

The  apogee  *  and  perigee  *  of  a  heavenly  body  express 
its  situation  with  regard  to  the  earth.  When  the  body  is 
nearest  to  the  earth,  it  is  said  to  be  in  perigee  ;  when  it  is 
furthest  from  the  earth,  it  is  said  to  be  in  apogee. 

Where  i»  the      1^50.  The  perihelia  of  the  planets,  an  seen  from 

•77-          j  the  sun,  are  in  the  following  signs  of  the  zodiac, 

perineiionana  namel       Mercury  in   Gemini,  Venus  in  Leo,  the 

Irthf  Earth  in  Cancer,  Mars  in  Pisces,  Vesta  in  Sagitta- 

rius, Juno  in  Taurus,  Ceres  in  Leo,  Pallas  in  Leo, 

Jupiter  in  Aries,  Saturn  in  Cancer,  Uranus  in  Virgo,  and  Neptune 

in  Taurus. 

What  is  meant  1251.  When  a  planet  is  so  nearly  on  a 

V»  the  inferior  ,.           .11               i           •,     ^ 

and  superior  une  wl^"  *ne  eai>th  and  the  sun  as  to  pass 

conjunction  and  between  them,  it  is  said  to  be  in  its  inferior 

opposition  of  a  .        ^  .             ,        11-1,1                •  .  •        •  i 

vianet  ?  conjunction  ;  when  behind  the  sun,  it  is  said 


*  The  jhiral  of  Perihelion  is  Perihelia,  and  of  Aphelion  is  Aphelia.  The 
words  perihelion,  aphelion,  apogee,  and  perigee,  are  derived  from  the  Greek 
language,  and  have  the  following  meaning  : 

Perihelion,  near  the  sun 

Aphelion,  from  the  sun. 

Perigee,  near  the  earth. 

Apogee,  fr<,m  the  earth 


350 


NATURAL    PHILOSOPHY. 


to  be  in   its  superior  conjunction ;  but  when  behind  the 
earth,  it  is  said  to  be  in  opposition. 

1252.  The   axes  of  the  planets,  in  their 
revolution  around  the  sun,  are  not  perpen- 
dicular to  thtir  orbits,  nor  to  the  plane  of  the 
ecliptic,  but  are  inclined  in  different  degrees 

1253.  This  is  one  of  the  most  remarkable 
circumstances  in  the  science  of  Astronomy, 
because  it  is  the  cause  of  the  different  seasons, 


What  is  the  in- 
clination of  the 
axes   of  the 
planets  to  the 
plane  of  their 
orbit  t  ? 


What  causes 
the  seasons  ? 
What  causes 
the  differences 
in  the  length  of 
the  days  and 
nights  ? 


spring,   summer,  autumn    and  winter ;  and 


because  it  is  also  the  cause  of  the  difference  in 
the  length  of  the  days  and  nights  in  the  different  parts  of 
the  world,  and  at  the  different  seasons  of  the  year. 

.  1254.  The  motion  of  the  heavenly  bodies  is  not  uniform. 
They  move  with  the  greatest  velocity  when  they  are  in 
perihelion,  or  in  that  part  of  their  orbit  which  is  nearest 
to  the  sun;  and  slowest  when  in  aphelion. 

1255.  It  was  discovered  by  Kepler,  and  proved  by 
Newton,  that  if  a  line  is  drawn  from  the  sun  to  either 
of  the  planets,  this  line 
passes  over  or  describes 
equal  areas  in  equal  times. 
This  line  is  called  the  ra- 
dius-vector. This  is  one  of 
Kepler's  great  laws. 

Explain  In     Fig.      185, 

Fig.  185.    let  g  represent  the 

sun,  and  E  the  earth,  and  the 
ellipse  or  oval,  be  the  earth's 
orbit,  or  path  around  the  sun. 
By  lines  drawn  from  the  sun 

at  S  to  the  outer  edge  of  the  «.„. 

figure,    the   orbit   is   divided 


ASTRONOMY.  351 

into  twelve  areas  of  different  shapes,  but  each  containing  the 
same  quantity  of  space.  Thus,  the  spaces  E  S  A,  A  S  B,  D  S  C, 
&c.,  are  all  supposed  to  be  equal.  Now,  if  the  earth  in  the 
space  of  one  month  will  move  in  its  orbit  from  E  to  A,  it  will 
in  another  month  move  from  A  to  B,  and  in  the  third  month 
from  B  to  C,  <fec.,  and  thus  its  radius  vector  will  describe  equal 
areas  in  equal  times. 

The  reason  why  the  earth  (or  any  other  heavenly  body)  moves 
with  a  greater  degree  of  velocity  in  its  perihelion  than  in  its 
aphelion  may  likewise  be  explained  by  the  same  figure.  Thus  : 

The  earth,  in  its  progress  from  F  to  L,  being  constantly  urged 
forward  by  the  sun's  attraction,  must  (as  is  the  case  with  a  fall- 
ing body)  move  with  an  accelerated  motion.  At  L,  the  sun's 
attraction  becomes  stronger,  on  account  of  the  nearness  of  the 
earth ;  and  consequently  in  its  motion  from  L  to  E  the  earth 
will  move  with  greater  rapidity.  At  E,  which  is  the  perihelion 
of  the  earth,  it  acquires  its  greatest  velocity.  Let  us  now  detain  it 
at  E,  merely  to  consider  the  direction  of  the  forces  by  which  it 
is  urged.  If  the  sun's  attraction  could  be  destroyed,  the  force 
which  has  carried  it  from  L  to  E  would  carry  it  off  in  the  dotted 
line  from  E  to  G,  which  is  a  tangent  to  its  orbit.  But,  while  the 
earth  has  this  tendency  to  move  towards  Gr,  the  sun's  attraction 
is  continually  operating  with  a  tendency  to  carry  it  to  S.  Now, 
when  a  body  is  urged  by  two  forces,  it  will  move  between  them , 
but,  as  the  sun's  attraction  is  constantly  exerted,  the  direction 
of  the  earth's  motion  will  not  be  in  a  straight  line,  the  diagonal 
of  one  large  parallelogram,  but  through  the  diagonal  of  a  num- 
ber of  infinitely  small  parallelograms  ;  which,  being  united,  form 
the  curve  line  E  A. 

It  is  thus  seen  that  while  the  earth  is  moving  from  L  to  E  the 
attraction  of  the  sun  is  stronger  than  in  any  other  part  of  its 
orbit,  and  will  cause  the  earth  to  move  rapidly.  But  in  its 
motion  fiom  E  to  A,  from  A  to  B,  from  B  to  C,  and  from 
C  to  F,  the  attraction  of  the  sun,  operating  in  an  opposite  direc- 
tion, will  cause  its  motion  from  the  sun  to  be  retarded,  until,  at 
F,  the  direction  of  its  motion  is  reversed,  imd  it  begin?  again  to 


NATURAL   PHILOSOPHY. 

approach  the  sun.  Thus  it  appears  that  in  its  passage  from  thd 
perihelion  to  the  aphelion  the  motion  of  the  earth,  as  well  as 
that  of  all  the  heavenly  bodies,  must  be  constantly  retarded, 
while  in  moving  from  their  aphelion  to  perihelion  it  is  con 
stantly  accelerated,  and  at  their  perihelion  the  velocity  will  be 
the  greatest.  The  earth,  therefore,  is  about  seven  days  longer 
in  performing  the  aphelion  part  of  its  orbit  than  in  traversing 
lie  perihelion  part  ;  and  the  revolution  of  all  the  other  planets 
being  the  result  of  the  same  cause,  is  affected  in  the  same  man- 
ner as  that  of  the  earth. 

What   are   the  1256'    The   °ther     tw°     Sreat     laWS     iis' 

three  laws  of     covered  by  Kepler,  on  which  the  discoveries 

^     '  of  Newton,  as   well  as  the  whole   modern 

theory  of  the  planets,  are  based,  are  — 

1257.  (1.)  That  the  planets  do  not  move  in  circles, 
but  in  ellipses,  of  which  the  sun  is  in  one  of  the  foci. 

1258.  (2.)  In  the  motion  of  the  planets,  the  squares 
of  the  times  of  revolution  are  as  the  cubes  of  the  mean 
distances  from  the  sun. 

It  was  by  this  law  that,  in  the  want  of  other  means,  the 
distance  of  the  planet  Uranus  from  the  sun  was  estimated. 
How  much  nearer  1259.  The  earth  is  about  three  millions 

mnl7ummerhe  of  miles  nearer  to  tbe  sun  in  wintcr  thar 
than  in  the  win-  in  summer.     The  heat  of  summer,  there- 


, 

in  this  question.}  the  distance  oi  the  earth  from  the  sun. 

The  sun  is  nearest  to  the  earth  in  the  summer  of  the  southern 
hemisphere,  but  the  heat  is  not  more  intense  there  than  in  cor- 
responding latitudes  of  the  north.  This  is  due  to  the  greater 
amount  of  land  in  the  northern  hemisphere,  which  by  its  radiat- 
ing power  heats  the  atmosphere  more  thoroughly. 

When  is  the  1260.   On  account  of  the  inclination  of 

heatofthe-sun    the   earth's   axis.-  the   rays  of  the   sun  fall 

more   or  less   obliquely   on   different  parts 


ASTRONOMY. 

rf  the  earth's  surface  at  different  seasons  of  the  year. 
The  heat  is  always  the  greatest  when  the  sun's  rays  fall 
vertically ;  and  the  more  obliquely  they  fall,  the"  fewer 
of  them  fall  on  any  given  space. 

This  is  the  reason  why  the  days  are  hottest  in  summer,  although 
the  earth  is  further  from  the  sun  at  that  time. 
Explain  1261.  Fig.  186  represents  the  manner  in  which 

the  rays  of  the  sun  fall  upon  the  earth  in  summer  and 
in  winter.  The  north  pole  of  the  earth,  at  all  seasons,  constantly 
points  to  the  north  star  N  ;  and,  when  the  earth  is  nearest  to  the 
sun,  the  rays  from  the  sun  fall  as  indicated  by  W  in  the  figure ; 

Fig.  186. 


and  as  their  direction  is  very  oblique,  the  amount  of  heat  for 
any  particular  area  of  this  portion  of  the  surface  is  much  less. 
Hence  we  have  cold  weather  when  the  earth  is  nearest  to  the  sun. 
But  when  the  earth  is  in  aphelion,  the  rays  fall  almost  vertically 
or  perpendicularly,  as  represented  by  S  in  the  figure ;  and  al- 
though the  earth  is  then  nearly  three  millions  of  miles  further 
from  the  sun,  the  heat  is  greatest,  because  the  rays  fall  more 
directly,  and  have  a  less  portion  of  the  atmosphere  to  traverse. 

This  may  be  more  familiarly  explained  by  comparing  summer 
rays  to  a  ball  or  stone  thrown  directly  at  an  object,  so  as  to 


354  NATURAL    PHILOSOPHY-. 

strike  it  witn  ill  its  force ;  and  winter  rays  to  the  same  ball  ci 
stone  thrown  obliquely,  so  as  merely  to  graze  the  object. 
Why  is  it  cooler  1262.  For  a  similar  reason  we  find,  even  in 
mornin*  than  8ummer>  taat  earlj  in  tne  morning  and  late  in 
in  the  middle  the  afternoon  it  is  much  cooler  than  at  noon, 
of  the  day  ?  because  the  sun  then  shines  more  obliquely. 

The  heat  is  generally  the  greatest  at  about  three  o'clock  in  the 
afternoon ;  because  the  earth  retains  its  heat  for  some  length  of 
time,  and  tht  additional  heat  it  is  constantly  receiving  from  tho 
sun  causes  an  elevation  of  temperature,  even  after  the  rays 
begin  to  fall  more  obliquely. 

What  causes  the  1263.  It  is  the  same  cause  which  occasions 
mUtfes^n  Differ-  the  varietJ  of  climate  in  different  parts  of  the 
ent  parts  of  the  earth.  The  sun  always  shines  in  a  direction 
world?  nearly  perpendicular,  or  vertical,  on  the  equator, 

and  with  different  degrees  of  obliquity  on  the  other  parts  of  the 
earth.  For  this  reason,  the  greatest  degree  of  heat  prevails  at 
the  equator  during  the  whole  year.  The  further  any  place  is 
situated  from  the  equator,  the  more  obliquely  will  the  rays  fall 
at  different  seasons  of  the  year,  and,  consequently,  the  greater 
will  be  the  difference  in  the  temperature. 

What  places  will  1264'  lf  the  axis  °f  the  earth  were  PerPen- 
nave  the  coolest  dicular  to  its  orbit,  those  parts  of  the  earth 
temperature?  which  lie  under  the  equator  would  be  constantly 
opposite  to  the  sun ;  and  as,  in  that  case,  the  sun  would,  at  all 
times  of  the  year,  be  vertical  to  those  places  equally  distant  from 
both  poles,  so  the  light  and  heat  of  the  sun  would  be  dispersed 
with  perfect  uniformity  towards  each  pole ;  we  should  have  no 
variety  of  seasons  ;  day  and  night  would  be  of  the  same  length, 
and  the  heat  of  the  sun  would  be  of  the  same  intensity  every 
day  throughout  the  year. 

What  effects  are  1265.  It  is,  therefore,  as  has  been  stated, 
inclination  of  *"'  owing"  to  the  inclination  of  the  earttis 
the  earth's  axis?  a3:ls  (/iaf  we  have,  the  agreeable  variety 


ASTRONOMY. 


of  the  seasons,  days  and  nights  of  different  lengths  ^ 
and  that  wisely  -ordered  variety  of  climate  which  causes 
S'O  great  a  variety  of  productions^  and  which  has  afford- 
ed so  powerful  a  stimulus  to  human  industry. 

12GG.  The  wisdom  of  Providence  is  frequently  displayed  in  appar- 
ent inconsistencies.  Thus,  the  very  circumstance  which,  to  the 
short-sighted  philosopher,  appears  to  have  thrown  an  insurmountable 
barrier  between  the  scattered  portions  of  the  human  race,  has  been 
wisely  ordered  to  establish  an  interchange  of  blessings,  and  to  bring 
the  ends  of  the  earth  in  communion.  Were  the  same  productions 
found  in  every  region  of  the  earth,  the  stimulus  to  exertion  would 
be  weakened,  and  the  wide  field  of  human  labor  would  be  greatly 
diminished.  It  is  our  mutual  wants  which  bind  us  together. 

1267.  In  order  to  understand  the  illustration  of  the  causes  of 
the  seasons,  &c.,  it  is  necessary  to  have  some  knowledge  of  the 
circles  which  are  drawn  on  the  artificial  representations  of  the 
earth.  It  is  to  be  remembered  that  all  ^f  these  circles  are 
wholly  imaginary;  that  is,  that  there  are  on  the  earth  itself  no 
such  circles  or  lines.  They  are  drawn  on  maps  merely  for  the 
purpose  of  illustration. 

Krp/ain  1268.  Fig.  187  represents  the  earth.     N  S  is  the 

axis,  or  Imaginary  line,  around  which  it  daily 


*°  '        ' 


Fig.  187. 


N  is  the  north  pole,  S  is  the  south  pole. 
These  poles,  it  will  be  seen,  aie  the 
extremities  of  the  axis  N  S.  CD 
represents  the  equator,  which  is  a  cir- 
cle around  the  earth,  at  an  equal  dis- 
tance from  each  pole.  The  curved 
lines  proceeding  from  N  to  S  are  me- 
ridians. They  are  all  circles  sur- 
rounding the  earth,  and  passing  through 
!he  poles.  These  meridians  may  be  multiplied  at  pleasure. 

The  lines  E  F,  I  K,  L  M,  and  G  H,  are  designed  to  reprcsei  t 
Circles  all  of  them  parallel  to  the  equator,  and  for  this  reason 
'.hey  are  called  parallels  of  latitude.  These  also  may  be  mul 
t'ipliei  at  pleasure. 

Bu*,  in  the  figure  the.se  lines,  which  are  parallel  to  the  equator, 
15* 


556  ^ATURAL    PHILOSOPHY. 

and  which  are  at  a  certain  distance  from  it,  have  a  different 
name,  derived  from  the  manner  in  which  the  sun's  rays  fall  on 
the  surface  of  the  earth. 

Thus  the  circle  I  K,  23^  degrees  from  the  equator,  is  called 
the  tropic  of  Cancer,  and  the  circle  L  M  is  called  the  tropic  oi 
Capricorn.  The  circle  E  F  is  called  the  Arctic  Circle.  It 
represents  the  limit  of  perpetual  day  when  it  is  summer  in  the 
northern  hemisphere,  and  of  perpetual  night  when  it  is  winter. 

On  the  21st  of  March  the  rays  of  the  sun  fall  vertically  on 
the  equator,  and  on  each  succeeding  day  on  places  a  little  to  the 
north,  until  the  21st  of  June,  when  they  fall  vertically  or  places 
23^  degrees  north  of  the  equator.  Their  vertical  direction  then 
turns  back  again  towards  the  equator,  where  the  rays  again  fall 
vertically  on  the  23d  of  September,  and  on  the  succeeding  days 
a  little  to  the  south,  until  the  21st  of  December,  when  they  fall 
vertically  on  the  places  23^  south  of  the  equator.  Their  verti- 
cal direction  then  again  turns  towards  the  equator  Hence  the 
circles  I  K  and  L  M  are  called  the  tropics  of  Cancer  and  Cap- 
ricorn. The  word  tropic  is  derived  from  a  word  which  signifies 
10  turn.  The  tropics,  therefore,  are  the  boundaries  of  the  sun's 
apparent  path  north  and  south  of  the  equator,  or  the  lines  at 
which  the  sun  turns  back.  .-  . 

The  circle  G  H  is  the  Antarctic  Circle,  and  represents  thu 
limit  of  perpetual  day  and  night  in  the  southern  hemisphere. 
The  line  L  K  represents  the  circle  of  the  ecliptic,  which,  as  has 
already  been  stated,  is  the  apparent  path  of  the  sun,  or  the  rea. 
path  of  the  earth.  This  circle,  although  it  is  generally  drawn 
on  the  terrestrial  globe,  is,  in  reality,  a  circle  in  the  heavens ; 
and  differs  from  the  zodiac  only  in  its  width,  —  the  zodiac  ex- 
tending eight  degrees  on  each  side  of  the  ecliptic. 
Explain  1269.  Fig.  188  represents  the  manner  in  which  the 

.  tg.  lo  .  gun  sh}nes  on  the  earth  in  different  parts  of  its  orbit , 
or,  in  other  words,  the  cause  of  the  change  in  the  seasons.  S 
represents  the  sun,  and  the  dotted  oval,  or  ellipse,  ABC  I),  the 
whit  of  the  earth.  The  outer  circle  represents  the  ?odiac  with 


ASTRONOMY. 

the  position  of  the  twelve  signs  or  constellations.  On  the 
of  June,  when  the  earth  is  at  D,  the  whole  northern  polar  reg:on 
is  continually  in  the  light  of  the  sun.  As  it  turns  on  its  axis, 
therefore,  it  will  be  day  to  all  the  parts  which  are  exposed  to 
the  light  of  the  sun.  But,  as  the  whole  of  the  Antarctic  circle 
is  within  the  line  of  perpetual  darkness,  the  sun  can  shine  on  no 
part  of  it.  It  will,  therefore,  be  constant  night  to  all  places 
within  that  circle.  As  the  whole  of  the  Arctic  circle  is 

Pig.  188. 


the  line  of  perpetual  light,  no  parr  of  that  circle  will  be  turned 
>rom  the  sun  while  the  earth  turns  on  its  axis.  To  all  placew, 
therefore,  within  the  Arctic  circle,  it  will  be  constant  day. 

On  the  22d  of  September,  when  the  earth  is  at  C,  its  axis  is 
aeither  inclined  io  nor  from  the  sun,  but  is  sidewise  ;  and,  o! 
course,  while  one-half  of  the  earth,  from  pole  to  pole,  is  enlight- 
ened, the  other  half  is  in  darkness,  as  would  be  the  case  if  its 
axis  were  perpendicular  to  the  plane  of  its  orbit;  and  it  is  this 


NATURAL   PHILOSOPHY. 

which  causes  the  days  and  nights  of  this  season  of  the  year  to? 
be  of  equal  length. 

On  the  23d  of  December  the  earth  has  progressed  in  its  orbit 
to  B,  which  causes  the  whole  space  within  the  northern  polar 
circle  to  be  continually  in  darkness,  and  more  of  that  part  of  th*1 
earth  north  of  the  equator  to  be  in  the  shade  than  in  the  light 
of  the  sun.  Hence,  on  the  21st  of  December,  at  all  places  north 
of  the  equator  the  days  are  shorter  than  the  nights,  and  at  all 
places  south  of  the  equator  the  days  are  longer  than  the  nights 
Hence,  also,  within  the  Arctic  circle  it  is  uninterrupted  night, 
the  sun  not  shining  at  all  ;  and  within  the  Antarctic  circle  it  is 
uninterrupted  day.  the  sun  shining  all  the  time. 

On  the  20th  of  March,  the  earth  has  advanced  still  further,  and 
is  at  A,  which  causes  its  axis,  and  the  length  of  the  days  and 
nights,  to  be  the  same  as  on  the  20th  of  September. 

„„  1270.  From   the  explanation  of  figure  198. 

What  is  meant 

by  the  Equinoxes  lfc  appears  that  there  are  two  parts  of  its  ore-it 

and  the  Sol-  Jn  which  the  days  and  nights  are  equal  all  ovei 
the  earth.  These  points  are  in  the  sign  of 
Aries  and  Libra,  which  are  therefore  called  the  equinoxes 
Aries  is  the  vernal  (or  spring)  equinox,  and  Libra  the  autumna? 
equinox. 

1271.  There  are  also  two  other  points,  called  solstices,  because 
the  sun  appears  to  stand  at  the  same  height  in  the  heavens  in 
the  middle  of  the  day  for  several  days.  These  points  are  in  the 
signs  Cancer  and  Capricorn.  Cancer  is  called  the  summer  sol- 
stice, and  Capricorn  the  winter  solstice. 

1272'   Da    and  niht  are  Caused  b     the  r°ta' 


How    are   day 

and  night  cans-  tion  of  the  earth   on  its  axis  every  24   houi  s. 

ed  and  what  is  It  is  d        to   that  gide  of  the   earth   which   k 

the  reason  of  the  . 

difference  in  towards  the  sun,  and  night  to  the  opposite  side. 

'heir  length.?  The  length  of  the  days  is  in  proportion  to  the 
inclination  of  the  axis  of  the  earth  towards  the  sun.  It  may  be  seen, 
by  the  above  figure,  that  in  summer  the  axis  is  most  inclined 
towards  the  sun,  and  then  the  days  are  the  longest.  A&  the  uortl* 


ASTitOA'UMY.  359 

pole  becomes  less  inclined,  the  days  shorten,  till  on  the  21st  of  De< 
eember  it  is  inclined  23 J-  degrees  frmn  the  sun,  when  the  day. 
are  the  shortest.  Thus,  as  the  earth  progresses  in  its  orbit,  after 
Lhe  days  are  the  shortest,  it  changes  its  inclination  towai  ds  the 
sun,  till  it  is  again  inclined  as  in  the  longest  days  in  the  summer. 
Which  of  the  1:>73.  As  the  difference  in  the  length  of  the 

^test^ffer-  da?S  and  the  nights>  and  the  change  °f  the 
ence  in  its  sea-  seasons,  &e.,  on  the  earth,  is  caused  by  the  in- 

sons  •  clination  of  the  earth's  axis,  it  follows  that  al] 

the  planets  whose  axes  are  inclined  must  experience  the  same 
vicissitude,  and  that  it  must  be  in  proportion  to  the  degree  of 
the  inclination  of  their  axes.  As  the  axis  of  the  planet  Jupiter 
is  nearly  perpendicular  to  its  orbit,  it  follows  that  there  can  be 
little  variation  in  the  length  of  the  days  and  little  change  in  the 
seasons  of  that  planet. 

1274.  There  can  be  little  doubt  that  some  of  the  other  planets 
and  satellites  are  inhabited;  and  although  it  may  he  thought 
that  some  of  them,  on  account  of  their  immense  distance  from  the 
dun,  experience  a  great  want  of  light  and  heat,  while  others  are  so 
near,  and  the  heat  consequently  so  great,  that  water  cannot  remain 
un  them  in  a  fluid  state,  yet,  as  we  see,  even  on  our  own  earth,  that 
creatures  of  different  natures  live  in  different  elements, —  as,  for 
instance,  fishes  in  water,  animals  in  air,  &c., —  creative  wisdom  could, 
undoubtedly,  adapt  the  being  to  its  situation,  and  with  as  little 
exertion  of  power  form  a  race  whose  nature  should  be  adapted  to 
the  nearest  or  the  most  remote  of  the  heavenly  bodies,  as  was  re- 
quired to  adapt  the  fowls  to  the  air,  or  the  fishes  to  the  sea. 

WhatiitheSton,        1275'  °F    THB    SUN.  —  The    Sun    is  a 
e-nd  what  is  its    spherical  body,  situated  near  the  centre  of 
gravity  of  the  system  of  planets  of  which 
our  earth  is  one. 

H^w  much  larger      -1276.  Its  diameter  is   853,000  English 
I*  the  earth  tJian  mjieg    which  js  equaj  to  ^Qg  diameters  of 
the  sun ».-,., 
\Answer  care-      the   earth ;   and,    as    spheres    are   to   each 

fuity-]  other  in  the  proportion  of  the  cube  of  their 

respective  diameters,  therefore  his  cubic  magnitude  must 
exceed  that  of  the  earth  one  million  of  times.  It 


$60  NATURAL    PHILOSOPHY. 

around  its  axis  in  25  days  and  3  hours.  This  has  been 
ascertained  by  means  of  several  dark  spots  which  have 
been  seen  with  telescopes  on  its  sui  face. 

127T.  Sir  Win.  Herschel  supposed  the  spots  on  the 
eun  to  be  the  dark  body  of  the  sun,  seen  through  open- 
ings in  the  luminous  atmosphere  which  surrounds  him. 

1278.  It  is  probable  that  the  sun,*  like  all  the  other 
heavenly  bodies  (excepting,  perhaps,   comets),  is   in- 
habited by  beings  whose  nature  is  adapted  to  their 
peculiar  circumstances. 

1279.  Many  theories  have  been  advanced  with  regard 
to  the  nature  of  the  sun.     By  some  it  has  been  regarded 
as  an  immense  ball  of  fire  ;  but  the  theory  which  seems 
most  in  accordance  with  facts  is,  that  the  light  and  heat 
are  communicated  from  a  luminous  atmosphere,  or  at- 
mosphere of  flame,  which  surrounds  the  sun,  at  a  con- 
siderable distance  above  the  surface. 

What  is  the  zo-  1280.  The  zodiacal  light  is  a  singular  phe- 
diacal  light,  and  nomeuoii,  accompanying  the  sun.  It  is  a  faint 
light  which  often  appears  .x>  stream  up  from 
the  sun  a  little  after  sunset  and  before  sunrise.  It  appears 
nearly  in  the  form  of  a  cone,  its  bides  being  somewhat  curved 
and  generally  but  ill  defined.  It  extends  often  from  50°  to  100° 
in  the  heavens,  and  always  nearly  in  the  direction  of  the  place 
of  che  ecliptic.  It  is  most  distinct  about  the  beginning  of  March, 
but  is  constantly  visible  in  the  torrid  zone.  The  cause  of  this 
phenomenon  is  not  known. 

1281.  The  sun,  as  viewed  from  the  different  planets,  appears 
>f  different  sizes  according  to  their  respective  distances.  Fig. 
189  affords  a  comparative  view  of  his  apparent  magnitude,  as 
seen  from  all  except  the  smaller  of  the  minor  planets. 

*  In  almanacs  the  sun  is  usually  represented  by  a  -small  circle,  with  the 
face  of  a  raun  in  it :  thus,  « 


ASTliONOMY. 


3(51 


Fig.  189. 


Apparent  Magnitude  uf  the  Sun  as  seen  from  thr>  Platiclf 


NATUBAL   PHILOSOPHY. 


Describe  the  128&    ^F    MERCURY.  —  Mercury    is    the 

planet  Mer-  Dearest  planet  to  the  sun,  and  is  seldom  seen; 
'ury'  because  his  vicinity  to  the  sun  occasions  his 

being  lost  in  the  brilliancy  of  tie  sun's  rays. 

How  many  1283.   The  heat  of  this  planet  is  so  great 

^thlplawt  that  water  cannot  exist  there  except  in  a 
Mercury  ?  state  of  vapor,  and  metals  would  be  melted. 
The  intensity  of  the  sun's  heat,  which  is  in  the  same  pro- 
portion as  its  light,  is  ,  seven  times  greater  in  Mercury 
than  on  the  earth,  so  that  water  there  would  be  carried 
off  in  the  shape  of  steam  ;  for,  by  experiments  made  with 
a  thermometer,  it  appears  that  a  heat  seven  times  greater 
than  that  of  the  sun's  beams  in  summer  will  make  water 
boil. 

,,     iii  1284.    Mercury,    although   in   appearance 

night  may  only  a  small  star,  emits  a  bright  white  iight, 
^een?Ty  **  ^  which  it  may  be  recognized  when  seen. 
It  appears  a  little  before  the  sun  rises,  and 
again  a  little  after  sunset  ;  but,  as  its  angular  distance 
from  the  sun  never  exceeds  twenty-three  degrees,  it  is 
never  to  be  seen  longer  than  one  hour  and  fifty  minutes 
after  sunset,  nor  longer  than  that  time  before  the  sun  rises. 

How  does  Mer-       1285.  When  viewed  through  a  good  tele- 

cury  appear       scope,   Mercury  appears  with  all  the  various 

when  seen  J  . 

through  a         phases,  or  increase  and  decrease  of  light,  with 

tslescope  ?  which  we  view  the  moon,  except  that  it  never 
appears  quite  full,  because  its  enlightened  side  is  turned 
directly  towards  the  earth  only  when  .the  planet  is  so  near 
the  sun  as  to  be  lost  to  our  sight  in  its  beams.  Like  that 
of  the  moon,  the  crescent  or  enlightened  side  of  Mercury 
is  always  towards  the  sun.  The  time  of  its  rotation  on  its 
axis  has  been  estimated  at  about  twenty-four  hours. 


A8TKONOMY.  #U# 

1286.    OF   VENUS.  —  Venus,    the   second 

planet  Venus.  p]anet  jn  or(Jer  from  the  smij  jg  faQ  neareat  to 
the  earth,  and  on  that  account  appears  to  be  the  largest 
and  most  beautiful  of  all  the  planets.  During  a  part  of 
the  year  it  rises  before  the  sun,  and  it  is  then  called  the 
morning  star ;  during  another  part  of  the  year  it  rises  after 
the  sun,  and  it  is  then  called  the  evening  star.  The  heat 
and  light  at  Venus  are  nearly  double  what  they  are  at  the 
earth. 

1287.  By  the  ancient  poets  Venus  was  called  Phosphor,  or  Luci- 
fer, when  it  appeared  to  the  west  of  the  sun,  at  which  time  it  is 
morning  star,  and  ushers  in  the  light  of  day  ;  and  Hesperus,  or 
Vesper,  when  eastward  of  the  sun,  or  evening  star. 

Why  is  Venus       1288.  Venus,  like  Mercury,  presents  to  us 

never  seen  late  all  the  appearances  of  increase  and  decrease 
at  night  ?  ,, , .  ,  ,  ,  n  , 

of  light  common  to  the  moon.     Spots  are  also 

sometimes  seen  on  its  surface,  like  those  on  the  sun.  By 
reason  of  the  great  brilliancy  of  this  planet,  it  may  some- 
'  imes  be  seen  even  in  the  day-time  by  the  naked  eye.  But 
t  is  never  seen  late  at  night,  because  its  angular  distance 
from  the  sun  never  exceeds  forty-five  degrees.  In  the 
absence  of  the  moon  it  will  cast  a  shadow  behind  an  opaque 
body. 

What  is  meant  1289.  Both  Mercury  and  Ven us  sometimes 
by  the  transit  pass  directly  between  the  sun  and  the  earth. 
ofapa  ^&  tkejr  illuminated  surface  is  towards  the  sun, 

their  dark  side  is  presented  to  the  earth,  and  they  appear 
like  dark  spots  on  the  sun's  disk.  This  is  called  the 
transit  of  these  planets. 

1290.  The  reason  why  we  cannot  see  the  stars  and  planets- in 
the  day-time  is,  that  their  light  is  so  faint  compared  with  the 
'ight  of  the  sun  reflected  by  our  atmosphere. 

Describe  the  1291.    OP    THE    EARTH.  —  The    Earth    OB 

Earth  as  a  .  .  ,          ..       .      .  .  .          , 

which  we  live  is  the  next  planet  in  the  solar 


#64  NATURAL    PHILObOPHY. 

system,  in  tkc  order  of  distance,  to  Venus.  It  is  a  large 
globe  or  ball,  nearly  eight  thousand  miles  in  diameter,  and 
about  twenty-five  thousand  miles  in  circumference.  It  is 
known  to  be  round,  — first,  because  it  casts  a  circular 
shadow,  which  is  seen  on  the  moon  during  an  eclipse ; 
secondly,  because  the  upper  parts  of  distant  objects  on 
its  surface  can  be  seen  at  the  greatest  distance ;  thirdly, 
it  has  been  circumnavigated.  It  is  situated  in  the  midst 
of  the  heavenly  bodies  which  we  see  around  us  at  night, 
and  forms  one  of  the  number  of  those  bodies  ;  and  it 
belongs  to  that  system  which,  having  the  sun  for  its  centre, 
and  being  influenced  by  its  attraction,  is  called  the  solar 
system. 

How  much  It  is  not  a  perfect  sphere,  but  its  figure  is 

longer  is  the     th  t  of  an   Mate   spheroid,   the   equatorial 

polar  than  the  .          .,        , 

equatorial  diameter  being  about  twenty -six  miles  longer 

diameter  of  the  than  its     olar  diameter. 

earth?  [Think 

before  you  It  is  attended  by  one  moon,  the  diameter 

speak.]  Of  which  is  about  two  thousand  miles.     Its 

mean  distance  from  the  earth  is  about  240,000  miles,  and 
it  turns  on  its  axis  in  precisely  the  same  time  that  it  per- 
forms its  revolution  round  the  earth ;  namely,  in  twenty- 
seven  days  and  seven  hours. 

1292.  The  earth,   when   viewed  from  the 

Describe  the 

earth  as  a         moon,  exhibits  precisely  the  same  phases  that 

moon.  tjie  moon  does  to  us,  but  in  opposite  order. 

When  the  moon  is  full  to  us,  the  earth  will  be  dark  to  the 
inhabitants  *  of  the  moon ;  and  when  the  moon  is  dark  to 
us,  the  earth  will  be  full  to  them.  The  earth  appears  to 
them  about  thirteen  times  larger  than  the  moon  does  to  us. 

*  This  observation  should  be  qualified  by  the  condition  that  the  moon  is 
mhitbitod.  Although  there  is  abundant  reason  for  the  belief  that  the 
pl'uiets  are  "  the  green  abodes  of  life,"  there  are  many  reasons  to  bel'eve 
thai"  the  moon,  in  Us  i-rent-nt  state,  i?  noitltor  inhabits!  n».r  hubitaUc 


ASTitONOMY.  365 

As  the  moon,  however,  always  presents  nearly  the  same 
side  to  the  earth,  there  is  one-half  of  the  moon  which  we 
never  see,  arid  from  which  the  earth  cannot  be  seen. 

1293.  As  this  book  may  possibly  incite  the  inquiry  how  it  is  th.it 
the  astronomer  is  ible  to  measure  the  size  and  distances  of  those 
•immense  bodies  tl.3  consideration  of  which  forms  the  subject  of 
Astronomy,  the  process  will  "here  be  described  by  which  the  diam- 
eter of  the  earth  may  be  ascertained. 

1294.  All  circles,  as  has  already  been  stated,  are  divided  into  36(1 
degr  es,  and,  by  means  of  instruments  prepared  for  the  purpose, 
the  c  imber  of  degrees  in  any  arc  or  part  of  a  circle  can  be  correctly 
ascertained.     Let  us  now  suppose  that  an  observer^  standing  upon 
any  fixed  point,  should  notice  the  position  of  a  particular  star,  —  the 
north  or  polar  star,  for  instance.     Let  him  then  advance  from  his 
station,  and  travel  towards  the  north,  until  he  has  brought  the  star 
exactly  one  degree  higher  over  his  head.     Let  him  then  measure  the 
distance  over  which  he  -has  travelled  between  the  two  points  of 
observation,  -and  that  distance  will  bo  exactly  the  length  of  one 
degree  of  the  earth's  circumference.     Let  him  multiply  that  dis- 
tance by  3GO,  and  it  will  give  him  the  circumference  of  the  earth. 
Having  thus  found  the  circumference,  the  diameter  may  readily  be 
found  by  the  common  rules  of  arithmetic. 

This  calculation  is  based  on  the  supposition  that  the  earth  is  a 
perfect  sphere,  which  is  not  the  case,  the  equatorial  diameter  being 
about  twenty^six  miles  longer  than  the  poJar.  But  it  is  sufficiently 
near  the  truth  for  the  present  purpose.  The  design  of  this  work 
not  admitting  rigid  mathematical  demonstrations,  this  instance  of 
the  commencement  of  a  calculation  is  given  merely  to  show  that 
what^he  astronomer  and  the  mathematician  tell  us,  wonderful  as 
it  may  appear,  is  neither  bare  assertion  nor  unfounded  conjecture. 

What  motions  1295.  It  has  been  stated  that  the  earth  re- 

have  the  inhabit-  volves  upon  its  axis  every  day.      Now,  as  the 

ants  of  t/ie  earth  ^    .       ,          t-»r  AAA      -i                         c 

tl      a~tl    sa  ear^n  1S  about  25,000  miles  in  circumierenct\ 

planet?  Sec,  it  follows  that  the  inhabitants  of  the  equator 
also,  No.  1296.  are  carrie(j  around  this  whole  distance  in  about 
twenty-four  hours,  and  every  hour  they  are  thus  cariied  through 
space  in  the  direction  of  the  diurnal  motion  of  the  earth  at  the 
rate  of  ^¥th  of  25,000  miles,  which  is  more  than  1000  miles  in 
an  hour. 

1296.  But  this  is  not  all.  Every  inhabitant  travels  with  the 
earth  through  its  immense  orbit,  the  diameter  of  which  is  about 


<J66  NATURAL   PHILOSOPHY. 

lions  of  miles  every  year.  This  will  give  him,  at  the  same  time, 
<!.  motion  of  more  than  68  000  miles  in  an  hour  in  a  different 
direction.  If  the  question  be  asked,  why  each  individual  is  not 
sensible  of  these  tremendously  rapid  motions,  the  answer  is, 
that  no  one  ever  knew  what  it  is  to  be  without  them.  We  can- 
not be  sensible  that  we  have  moved  without  feeling  our  motion, 
as  when  in  a  boat  a  current  takes  us  in  one  direction,  while  a 
gentle  wind  carries  us,  at  the  same  time,  in  another  direction. 
It  is  only  when  our  progress  is  arrested  by  obstacles  of  some 
kind  that  we  can  perceive  the  difference  between  a  suite  oi 
motion  and  a  state  of  rest. 

What  ivould  1297.  The  rapid  motion  of  a  thousand  miles  in 
be  the  conse-  an  hour  is  not  sufficient  to  overcome  the  centri- 

yuenceifthe      petaj  force  caused  by  gravity;  but,  if  the  earth 

earth  should       \     . ,  /  ' 

revolve  on  its     should  revolve  around  its  axis  seventeen  times  in 

axis  once  in  a  day,  instead  of  once,  all  bodies  at  the  equator 
an  hour  1  WQuld  be  lifted  ^  and  ^  attractjon  Of  gravita- 

tion would  be  counterbalanced,  if  not  wholly  overcome. 

1298.  Certain  irregularities  in  the  orbit  of  the  earth  have 
been  noticed  by  astronomers,  which  show  that  it  is  deviating 
from  its  elliptical  form,  and  approaching  that  of  a  circle.  In 
this  fact,  it  has  been  thought,  might  be  seen  the  seeds  of  decay. 
But  Laplace  has  demonstrated  that  these  irregularities  proceed 
from  causes  which,  in  the  lapse  of  immensely  long  periods, 
counterbalance  each  other,  and  give  the  assurance  that  there  is 
no  other  limit  to  the  present  order  of  the  universe  than  the  will 
of  its  great  Creator. 

Describe  the  1299.  OF  MARS. — Next  to  the  earth  is 
planet  Mars,  the  planet  Mars.  It  is  conspicuous  for  its 
fiery-red  appearance,  which  is  supposed  by  Sir  John 
Itersclier*  to  be  caused  by  the  color  of  its  soil. 

*  Sir  John  Hcrschcl  is  the  son  of  Sir  William  Herschei,  the  discovo.r^r 
oi'  *he  planet  Uraruw. 


ASTRONOMY.  36? 

The  degree  of  heat  and  light  at  Mars  is  less  than  half  of 
that  received  by  the  earth. 

1300.  OF  THE  MINOR  PLANETS. — It.has  already  been  mention 
ed  that  between  the  orbits  of  Mars  and  Jupiter  one  hundred  and 
thirteen  small  bodies  have  been  discovered,  which  are  called  the 
minor  planets.  It  is  a  remarkable  fact,  that  before  the  discovery 
of  Bode's  law  (see  No.  1232)  certain  irregularities  observed  in  the 
motions  of  the  old  planets  induced  some  astronomers  to  sup- 
pose that  a  planet  existed  between  the  orbits  of  Mars  ar.d  Jupi- 
tei.  The  opinion  has  been  advanced  that  these  small  bodies 
originally  composed  one  larger  one,  which,  by  some  unknown 
force  or  convulsion,  burst  asunder.  This  opinion  is  maintained 
with  much  ingenuity  and  plausibility  by  Sir  David  Brewster. 
(See  Edin.  Encyc.,  art.  ASTRONOMY.)  Dr.  Brewster  further 
supposes  that  the  bursting  of  this  planet  may  have  occasioned 
"the  phenomena  of  meteoric  otones ;  that  is,  stones  which  have 
fallen  on  the  earth  from  the  atmosphere. 

Describe  the  1301.  OF  JUPITER.  — Jupiter  is  the  largest 
planet  Jupiter.  pjanet  Of  tne  solar  system,  arid  the  most  bril- 
liant, except  Venus.  The  heat  and  light  at  Jupiter  art 
about  twenty-five  times  less  than  that  at  the  earth.  This 
planet  is  attended  by  four  moons,  or  satellites,  the  shadows 
of  some  of  which  are  occasionally  visible  upon  his  surface. 

1302.  The  distance  of  -those  satellites  from  the  planet  are 
two,  four,  six  and  twelve  hundred  thousand  miles,  nearly. 

The  nearest  revolves  around  the  planet  in  less  than  two  days ; 
the  next,  in  less  than  four  days ;  the  third,  in  less  than  eight 
days ;  and  the  fourth,  in  about  sixteen  days. 

These  four  moons  must  afford  considerable  light  to  the  inhab- 
itants of  the  planet ;  for  the  nearest  appears  to  them  four  times 
the  size  of  our  moon,  the  second  about  the  same  size,  the  third 
somewhat  less,  and  the  fourth  about  one-third  the  diameter  of 
our  moon. 


363  NATURAL 

1303.  As  the  axis  of  Jupiter  is  nearly  perpendicular  to  its 
orbit,  it  has  no  s-ensible  change  of  season?. 

1304.  The  satellites  of  Jupiter  often  pass  be« 
What  use  has 
been  made  of     nm(i  tne  body  of  the  planet,  and  also  into  its 

the  ecfipses  of  shadow,  and  are  eclipsed.  These  eclipses  are  of 
,-£  j'  use  in  ascertaining  the  longitude  of  places  on  the 

earth.  By  these  eclipses,  also,  it  has  been  ascer- 
tained that  light  is  about  eight  minutes  in  coming  from  the  sun 
to  the  earth ;  for  an  eclipse  of  one  of  these  satellites  appears 
to  us  to  take  place  sixteen  minutes  sooner  when  the  earth  is  in 
that  part  of  its  orbit  nearest  Jupiter  than  when  in  the  part 
furthest  from  that  planet.  Hence,  light  is  sixteen  minutes  in 
crossing  the  earth's  orbit,  and  of  course  half  of  that  time,  01 
eight  minutes,  in  coming  from  the  sun  to  the  earth. 
What  is  the  ap-  1305.  When  viewed  through  a  telescope, 

"viler"™  seen  *""  several  belts  or  bands  are  distinctly  seen,  some- 
through  a  tele-  times  extending  across  his  disk,  and  sometimes 
scope?  interrupted  and  broken.  They  differ  in  dis- 

tance, position,  and  number.  They  are  generally  dark;  but 
white  ones  have  been  seen. 

On  account  of  the  immense  distance  of  Jupiter  from  the  sun 
and  also  from  Mercury,  Venus,  the  Earth  and  Mars,  observer? 
on  Jupiter,  with  eyes  like  ours,  can  never  see  either  of  the  a^ove 
named  planets,  because  they  would  always  be  immersed  in  the 
sun's  rays. 

Describe  the  1306.    OF  SATURN. — Saturn  is  the  sec- 

planet  Saturn*  ond  in  size,  and  the  last  but  two  in  dis- 
tance from  the  sun.  The  degree  of  heat  and  light  al 
this  planet  is  eighty  times  less  than  that  at  the  earth. 
How  is  Saturn  1307.  Saturn  is  distinguished  from  the 
particularly  other  planets  by  being  encompassed  bv 
t^ee  large  luminous  rings.  They  reflect 
the  sun's  light  in  the  same  manner  as  his 
moons.  They  are  entirely  detached  from  each  other,  ard 


from  the  body  of  the  planet.  They  turn  on  nearly  the 
'game  axis  with  the  planet,  and  in  nearly  the  same  tiino 

1308.  These  rings  move  together  around  the  planot, 
but  are  about  three  minutes  longer  in  performing  their 
revolution  about  him  than  Saturn  is  in  revolving  about 
his  axis.    The  edge  of  these  rings  is  constantly  at  right 
angles  with  the  axis  of  die  planet.     Stars  are  said  to 
have  been  seen  between  the  rings,  and  also  between  the 
inner  ring  and  the  body  of  the  planet.     The  breadth  of 
the  two  outer  rings  is  about  57,000  miles,  and  the  dis- 
tance of  the  second  ring  from  the  planet  is  about  19,000 
miles.    As  they  cast  shadows  on  the  planet,  Sir  AViu 
Herschel  thought  them  solid. 

1309.  The  surface  of  Saturn  is  sometimes  diversified, 
like  that  of  Jupiter,  with  spots  and  belts.    Saturn  has 
eight  satellites,  or  moons,  revolving  around  him  at  dif- 
ferent distances,  and  in  various  times,  from  less  than 
one  to  eighty  days.   • 

1310.  Saturn  may  be  known  by  his  pale  and  steady  light. 
The  eight  moons  of  Saturn  revolve  at  different  distances  around 
the  outer  edge  of  his  rings.     Sir  William  Herschel  saw  them 
moving  along  it,  like  bright  beads  on  a  white  string.     They  do 
not  often  suffer  eclipse  by  passing  into  the  shadow  of  the  planet, 
because  the  ring  is  in  an  oblique  direction. 

Describe  the  1311.  OF  URANUS. — Uranus,  the  fourth 
planetUranus.  in  size,  is  the  most  remote  of  all  the  old 
planets.  It  is  scarcely  visible  to  the  naked  eye.  Tho 
light  and  heat  at  Uranus  are  about  360  times  less  than 
that  of  the  earth. 

1312.  This  planet  was  long  known  by  the  name  of 
Ilcrschel,  the  discoverer,  who,  in  announcing  his  dis- 
covery, named  it  the  "  Georgiura  Sidus,"  in  honor  ot 
King  George  III.  The  name  of  Uranus  was  given  to  it 
by  the  continental  astronomers. 


370  NATURAL   PHILOSOPHY. 

It  was  formerly  considered  a  small  star,  but  Sir  William 
Herschel,  in  1781,  discovered,  from  its  motion,  that  it  is  a 
planet. 

By  how  many  1313'   IJranuS  is  attended  bJ  foUT  moons> 

moons  is  Uranus  or  satellites,  all  of  which  were  discovered 

by  Sir  William  Herschel,  and  all  of  them 
revolve  in  orbits  nearly  perpendicular  to  that  of  the  planet. 
Their  motion  is  retrograde. 

w,      .  1314.   It  appears  to  be  a  general  law  of  sat- 

general  law  of  ellites,  or  moons,  that  they  turn  on  their  axis 
the  rotation  of  ^n  tjie  same  ^me  in  w]iicfi  they  revolve  around 
satellites  f 

their  primaries.  On  this  account,  the  inhabit- 
ants of  secondary  planets  observe  some  singular  appearances, 
which  the  inhabitants  of  primary  planets  do  not.  Those  who 
dwell  on  the  side  of  a  secondary  planet  next  to  the  primary  will 
always  see  that  primary ;  while  those  who  live  on  the  opposite 
Fide  will  never  see  it.  Those  who  always  see  the  primary  will 
see  it  constantly  in  very  nearly  the  same  place.  For  example, 
those  who  dwell  near  the  edge  of  the  moon's  disk  will  always 
gee  the  earth  near  the  horizon,  and  those  in  or  near  the  centre 
will  always  see  it  directly  or  nearly  overhead.  Those  who  dwell 
in  the  moon's  south  limb  will  see  the  earth  to  the  northward , 
those  in  the  north  limb  will  see  it  to  the  southward;  those  in 
the  east  limb  will  see  it  to  the  westward ;  while  those  in  the 
west  limb  will  see  it  to  the  eastward  ;  and  all  will  see  it  nearer 
the  horizon  in  proportion  to  their  own  distance  from  the  centre 
of  the  moon's  disk.  Similar  appearances  are  exhibited  to  the 
inhabitants  of  all  secondary  planets.  These  observations  are 
predicated  on  the  supposition  that  the  moon  is  inhabited.  But 
it  is  not  generally  believed  that  our  Tnoon  is  inhabited,  or  in  its 
present  condition  fitted  for  the  residence  of  any  class  of  beings 
1315.  It  is  a  singular  circumstance,  that  before  the  discovery  tf 


ASTRONOMY.  37l 

CTraims  some  d.sturbances  and  deviations  were  observed  hy  astron. 
jniers  in  the  motions  of  Jupiter  and  Saturn,  which  they  could 
account  for  onl  y  on  the  supposition  that  these  two  planets  were  in 
duenced  by  the  attraction  of  some  more  remote  and  undiscovered 
planet.  The  discovery  of  Uranus  completely  verified  their  opinions. 
and  shows  the  extreme  nicety  with  which  astronomers  observe  the 
motions  of  planets. 


What  led  to  the  ^^®  ^F  NEPTUNE.  —  The  discovery  of  the 
discovery  of  the  planet  Neptune  (named  originally  Le  Verrier, 
planet  Neptune  ?  fi.om  itg  discovererj  in  1846)  is  one  of  the  greatest 

triumphs  which  the  history  of  science  records.  As  certain  per- 
turbations of  the  movements  of  Saturn  led  astronomers  to  sus- 
pect the  existence  of  a  remoter  planet,  which  suspicions  were 
fully  confirmed  in  the  discovery  of  Uranus,  so  also,  after  the  dis- 
covery of  Uranus,  certain  irregularities  were  perceived  in  his 
motiong,  that  led  the  distinguished  astronomers  of  the  day  to  the 
belief  that  even  beyond  the  planet  Uranus  still  another  undis- 
covered planet  existed,  to  reward  the  labors  of  the  discoverer. 
Accordingly  Le  Verrier,  a  young  French  astronomer,  urged  by 
his  friend  A  tag©,  determined  to  devote  himself  to  the  attempt 
at  discovery.  With  indefatigable  industry  he  prepared  new 
tables  of  planetary  motions,  from  which  he  determined  the  pertur- 
bations of  the  planets  Jupiter,  Saturn,  and  Uranus,  and  as  early 
as  June,  1846,  in  a  paper  presented  to  the  Academy  of  Sciences 
in  Paris,  he  pointed  out  where  the  suspected  planet  would  be 
on  the  1st  of  January,  1847.  He  subsequently  determined  the 
mass  and  the  elements  of  the  orbits  of  the  planet,  and  that,  too,  be- 
fore it  had  been  seen  by  a  human  eye.  On  the  18th  of  September 
of  1846,  he  wrote  to  his  friend,  M.  Galle,  of  Berlin,  requesting 
him  to  direct  his  telescope  to  a  certain  point  in  the  heavens,  where 
he  suspected  the  stranger  to  be.  His  friend  complied  with  his 
request,  and  on  the  first  evening  of  examination  discovered  a 
strange  star  of  the  eighth  magnitude,  which  had  not  been  laid  down 
in  any  of  the  maps  of  that  portion  of  the  heavens.  The  follow- 
ing evening  it  was  found  to  have  moved  in  a  direction  and  with  a 
velocity  very  nearly  like  that  which  Le  Verrier  had  pointed  out 
Che  planet  wats  found  within  less  than  one  degree  of  the  place 
16 


J72  NATUKA.L    PHILOSOPHY. 

where  Le  Verrier  had  located  it.  It  was  subsequently  ascer- 
tained that  a  young  English  mathematician,  Mr.  Adams,  of 
Cambridge,  had  been  engaged  in  the  same  computations,  and 
had  arrived  at  nearly  the  same  results  with  Le  Verrier. 

1317.  What  shall  we  say  of  science,  then,  that  enables  its  devoted 
followers  to  reach  out  into  space,  and  feel  successfully  in  the  dark 
for  an  object  more  than  twenty-eight  hundred  millions  of  iniJes 
distant  ? 

1318.  In  conclusion  of  this  brief  notice  of  the  planets,  a  plate 
is  here  presented  showing  the  relative  appearance  of  the  planets 
is  viewed  through  a  telescope.     It  will  be  observed  that  the 
planets  Mercury  and  Venus  have  similar  phases  to  those  of  our 
uioon. 

Pig   190 


Relative  Telescopic  appearance  of  the  Planets. 


,5  a  1319.  OF  COMETS.  —  The  word  Comet  is  de- 
?  rived  from  a  Greek  word,  which  means  hair;  and 
this  name  is  given  to  a  numerous  class  of  bodies,  which  occa- 
sionally visit  and  appear  to  belong  to  the  solar  system.  These 
bodies  seem  to  consist  of  a  nucleus,  attended  with  a  lucid 
haze,  sometimes  resembling  flowing  hair  ;  from  whence  the 
aame  is  derived.  Some  comets  appear  to  consist  wholly 


ASTKONOMY.  373 

af  tftis  hazy  or  hairy  appearance,  which  is  frequently  ealloci 
the  tail  of  the  comet. 

Fig.  191 


Comet  of  1811,  one  of  the  most  brilliaut  of  modern  times.     Period,  2888 

years. 

1320.  In  ancient  times  the  appearance  of  comets  was  regard- 
ed with  superstitious  fear,  in  the  belief  that  they  were  the  fore- 
runners of  some  direful   calamity.     These  fears  have  now  been 
banished,  and  the  comet  is  viewed  as  a  constituent  member  of 
the  system,  governed  by  the  same  harmonious  and  unchanging 
laws  which  regulate  and  control  a     the  other  heavenly  bodies. 

1321.  The  number  of  comets  that  have  occasionally  appeared 
within  the  limits  of  the  solar  system  is  variously  stated  from  350 
to  500.     The  paths  or  orbits  of  about  98  of  these  have  been 
calculated  from  observation  of  the  times  at  which  they  most 
nearly  approached  the  sun ;  their  distance  from  it  and  from  the 
earth  at  those  times ;  the  direction  of  their  movements,  whether 
from  cae-t  to  wet?t,  or  from  we?t  to  east ;  and  the  places  in  tli3 


374  NATURAL   P11ILOSOPII1. 

starry  sphere  at  which  their  orbits  crossed  that  of  the  earth  and 
their  inclination  to  it.  The  result  is,  that,  of  these  98,  24  passed 
between  the  sun  and  Mercury.  33  passed  between  Mercury  and 
Venus,  21  between  Venus  and  the  Earth,  16  between  the  Earth 
and  Mars,  3  between  Mars  and  Ceres,  and  1  between  Ceres  and 
Japiter  :  that  50  of  these  comets  moved  from  east  to  west ;  that 
fcheir  orbits  were  inclined  at  every  possible  angle  to  that  of  the 
earth.  The  greater  part  of  them  ascended  above  the  orbit  of 
the  earth  when  very  near  the  sun ;  and  some  were  observed  to 
dash  down  from  the  upper  regions  of  space,  and,  after  turning 
round  the  sun,  to  mount  again. 

1322.  Comets,  in  their  revolution,  describe 
What  is  the  shape 
of  the  orbits  of     long  narrow  ovals.     They  approach  very  near 

comets  ?  the  sun  in  one  of  the  ends  of  these  ovals,  and 

when  they  are  in  the  opposite  end  of  the  orbit  their  distance 
from  the  sun  is  immensely  great. 

1323.  The  extreme  nearness  of  approach  to  the  sun  gives  to 
a.  comet,  when  in  perihelion,  a  swiftness  of  motion  prodigiously 
great.     Newton  calculated  the  velocity  of  the  comet  of  1680  to 
be  880,000  miles  an  hour.     This  comet  was  remarkable  for  its 
near  approach  to  the  sun,  being  no  further  than  580,000  miles 
from  it,  which  is  but  little  more  than  half  the  sun's  diameter 
Brydone  calculated  that  the  velocity  of  a  comet  which  he  ob- 
served at  Palermo,  in  1V70,  was  at  the  rate  of  two  millions  and 
a  half  of  miles  in  an  hour. 

1324.  The  luminous  stream,  or  tail,  of  a  comet,  follows  it  aa 
it  approaches  the  sun,  and  goes  before  it  when  the  3omet  recedes 
from  tho  sun.     Newton,  and  some  other  astronomers,  considereo 
the  tails  of  comets  to  be  vapors,  produced  by  the  excessive  heat 
of  the  sun.     Others  have  supposed  them  to  be  caused  by  a  re- 
pulsive influence  of  the  sun.     Of  whatever  substance  they  may 
be,  it  is  certain  that  it  is  very  rare,  because  the  stars  may  bo 
distinctly  seen  through  it. 

1325    The  tails  of  comets  differ  very  greatly  in  length, 


&8T1CONOM1'. 


and  some  are  attended  apparently  by  only  a  small  cloudy 
light,  while  the  length  of  the  tail  of  othsrs  has  been  esti- 
mated at  from  50  to  80  millions  of  miles. 


Kg.  192 


The  comet  of  1680,  observed  by  Newton.  Eapidity  of  its  motion  around 
the  sun,  a  million  of  miles  in  an  hour. 

Length  of  tail,  100  millions  of  miles.  Period,  600  years.  It  has  never  re- 
appeared. 

1326.  It  has  been  argued  that  comets  consist  of  very  little 
solid  substance,  because,  although  they  sometimes  approach  very 
near  to  the  other  heavenly  bodies,  they  appear  to  exert  no  sensi- 
ble attractive  force  upon  those  bodies.  It  is  said  that  in  1454 
the  moon  was  eclipsed  by  a  comet.  The  comet  must,  therefore, 
have  been  very  near  the  earth  (less  than  240,000  miles) ;  yet  it 
produced  no  sensible  effect  on  the  earth  or  the  moon  ;  for  it  did 
aoi  cause  them  to  make  any  perceptible  deviation  from  their 


37<3  NATURAL   PHILOSOPHY. 

accustomed  paths  round  the  sun.  It  has  been  ascertained  tlt&i 
comets  are  disturbed  by  the  gravitating  power  of  the  planets  ; 
but  it  does  not  appear  that  the  planets  are  in  like  manner  affected 
by  comets. 

Some  comets  have  exhibited  the  appearance  of  two  or  raoiv 
tails,  and  the  great  comet  of  1744  had  six 

Fig    193 


The  great  comet  of  1744. 


ASTRONOMY. 


377 


J8427.  Many  comets  escape  observation  because  tb-.y  traverse 
fhat  part  of  the  heavens  only  which  is  above  the  borizon  in  the 
Jay-time.  They  are.  therefore,  lost  in  the  brilliancy  of  the  sun. 
and  can  be  seen  only  when  a  total  eclipse  of  the  sun  takes  place. 
Seneca,  60  years  before  the  Christian  Era,  states  that  a  large 
comet  was  actually  observed  very  near  the  sim,  during  an  eclipse 

1328.  Dr.  Halley,  Professor  Encke  and  Gambart,  are  the  first 
astronomers  that  ever  successfully  predicted  the  return  of  a 
comet.  The  periodical  time  of  Halley's  comet  is  about  76  years. 
It  appealed  last  in  the  fall  of  1835,  and  presented  diilererit  aj> 


Halley's  comet,  as  seen  by  (Sir  Joiiu  iierscheJ,  October  29th,  1835.  Da- 
changeable  in  its  appearance.  First  recognized  by  Halley  in  1682.  Period, 
16  years. 


378 


^"ATUKAL    PHILOSOPHY. 


pearances  from  (113*61601  points  of  observation.  That  of  Enck« 
is  about  1200  days ;  tliat  of  Biela,  about  6£  years.  This  last 
comet  appeared  in  1832  and  in  1838. 

Pig.  195. 


Halley's  comet,  as  seen  by  Struve,  Oct.  12th,  1835. 
in  1682.    Period,  75  years. 


First  seen  by  Halley 


1329.  The  comet  of  1758,  the  return  of  which  was  predicted 
oy  Dr.  Halley,  was  regarded  with  great  interest  by  astronomers, 
because  its  return  was  predicted.  But  four  revolutions  before, 
in  1456,  it  was  looked  upon  with  the  utmost  horror.  Its  long 
tail  spread  consternation  over  all  Europe,  already  terrified  by 
the  rapid  success  of  the  Turkish  arms.  Pope  Callixtus,  on  this 
occasion,  ordered  a  prayer,  in  which  both  the  comet  and  the 
Turks  were  included  in  one  anathema.  Scarcely  a  year  or  a 


ASTRONOMY.  37  y 

month  now  elapses  without  the  appearance  of  a  comet  in  our 
system.  But  it  is  now  known  that  they  are  bodies  of  such  ex- 
treme rarity  that  our  clouds  are  massive  in  comparison  with 
them.  They  have  no  more  density  than  the  air  under  an  ex- 
nausted  receiver.  Herschel  saw  stars  of  the  6th  magnitude 
through  a  thickness  of  30,000  miles  of  cometic  matter.  The 
number  of  comets  in  existence  within  the  compass  of  the  solai 
system  is  stated  by  some  astronomers  as  over  seven  millions. 

1330.  Fig.  194  represents  Halley's  comet,  as  seen  by  Sir  John 
Herschel,  while  Fig.  195  represents  the.  same  comet  as  seen  only 
a  few  days  before  by  Struve. 

1331.  THE  COMET  or  1856.— ^  The  following  interesting  details  in 
relation  to  a  comet  expected  in  1856  were  given  by  Babinet,  an  em- 
inent  French  astronomer.     It   is  translated  from  the  Courier  des 
Etats   Unis. 

"  This  comet  is  one  of  the  grandest  of  which  historians  make 
mention.  Its  period  of  revolution  ia  about  three  hundred  years.  It 
was  seen  in  the  years  104,  392,  683,  975,  1264,  and  the  last  time  in 
1556.  Astronomers  agreed  in  predicting  its  return  in  1848 ;  but  it 
failed  to  appear,  and  continues  to  shine  still  unseen  by  us.  Already 
the  observatories  began  to  be  alarmed  for  the  fate  of  their  beautiful 
wandering  star,  when  a  learned  calculator  of  Middlebourg,  M. 
Bomrne,  reassured  the  astronomical  world  of  the  continued  existence 
of  the  venerable  and  magnificent  comet. 

"•  Disquieted,  as  all  other  astronomers  were,  by  the  non-arrival 
of  the  comet  at  the  expected  time,  M.  Bomme,  aided  by  the  prepar- 
atory labors  of  Mr.  Hind,  has  revised  all  the  calculations  and  esti- 
mated all  the  actions  of  all  the  planets  upon  the  comet  for  three 
hundred  years  of  revolution.  The  result  of  this  patient  labor  gives 
the  arrival  of  the  comet  in  August,  1858,  with  an  uncertainty  of 
two  years,  more  or  less  ;  so  that  from  1856  to  1860  we  may  expect 
the  great  comet  which  was  the  cause  of  the  abdication  of  the  Em- 
peror Charles  V.,  in  1556. 

"  It  is  known  that,  partaking  of  the  general  superstition,  which 
interpreted  the  appearance  of  a  comet  as  the  forerunner  of  som** 
fatal  event,  Charles  V.  believed  that  this  comet  addressed  its  menaces 
particularly  to  him,  as  holding  the  first  rank  among  sovereigns.  The 
great  and  once  wise  but  now  wearied  and  shattered  monarch,  had 
been  for  some  time  the  victim  of  cruel  reverses.  There  were  threat- 
ening indications  in  the  political,  if  not  in  the  physical  horizon,  of  a 
still  greater  tempest  to  come.  lie  was  left  to  cry  in  despair,  (  For- 
tune abandons  old  men.'  The  appearance  of  the  blazing  star  seemed 
Co  him  an  admonition  from  Heaven  that  he  must  cease  to  bo  a  sov- 
if  he  would  avoid  a  fatality  from  which  one  without  author- 

16* 


380  NATURAL    PHILOSOPHY. 

ity  might  be  spared.  It  is  known  that  the  emperor  survived  his 
abdication  but  a  little  more  than  two  years. 

'*  Another  comet,  which  passed  near  us  in  1835,  and  which  has 
appeared  25  times  since  the  year  13  before  the  Christian  Era,  hari 
been  associated  by  the  (superstitious  with  many  important  events 
which  have  occurred  near  the  periods  of  its  visitation. 

"  In  1066,  William  the  Conqueror  landed  in  England  at  the  head 
of  a  numerous  army  about  the  time  that  the  comet  appeared  which 
now  bears  the  name  of  Halley's  comet.  The  circumstance  was 
regarded  by  the  English  as  a  prognostic  of  the  victory  of  the  Nor- 
mans. It  infused  universal  terror  into  the  rninds  of  the  people,  and 
contributed  not  a  little  towards  the  submission  of  the  country  after 
the  battle  of  Hastings,  as  it  had  served  to  discourage  the  soldiers 
of  Harold  before  the  combat.  The  comet  is  represented  upon  the 
famous  tapestry  of  Bayeux,  executed  by  Queen  Matilda,  the  wife  of 
the  conqueror. 

"  This  celebrated  tapestry  is  preserved  in  the  ancient  episcopal 
palace  at  Bayeux.  It  represents  the  principal  incidents,  including 
the  appearance  of  the  comet,  in  the  history  of  the  conquest  of  Eng- 
land by  William,  Duke  of  Normandy.  It  is  supposed  to  have  been 
executed  by  Matilda,  the  conqueror's  wife,  or  by  the  Empress  Ma- 
tilda, daughter  of  Henry  I.  It  consists  of  a  linen  web,  214  feet  in 
length  and  20  inches  broad  ;  and  is  divided  into  72  compartments, 
each  having  an  inscription  indicating  its  subject.  The  figures  ara 
all  executed  by  the  needle. 

"  The  same  comet,  in  1451,  threw  terror  among  the  Turks  under 
the  command  of  Mahomet  II.,  and  into  the  ranks  of  the  Christians 
during  the  terrible  battle  of  Belgrade,  in  which  forty  thousand  Mus- 
sulmans perished.  The  comet  is  described  by  historians  of  the  time 
as  '  immense,  terrible,  of  enormous  length,  carrying  in  its  train  a 
tail  which  covered  two  celestial  signs  (60  degrees),  and  producing 
universal  terror.'  Judging  from  this  portrait,  comets  have  singu- 
larly degenerated  in  our  day.  It  will  be  remembered,  however,  that 
in  1811  there  appeared  a  comet  of  great  brilliancy,  which  inspired 
some  superstitious  fears.  Since  that  epoch  science  has  noted  nearly 
30  comets,  which,  with  few  exceptions,  were  visible  only  by  the  aid 
of  the  telescope.  Kepler,  when  asked  how  many  comets  he  thought 
there  were  in  the  heavens,  answered,  '  As  many  as  there  are  fish  in 
the  sea.' 

"  Thanks  to  the  progress  of  astronomical  science,  these  singular 
8tars  are  no  longer  objects  of  terror.  The  theories  of  Newton, 
Halley,  and  their  successors,  have  completely  destroyed  the  imag- 
inary empire  of  comets.  As  respects  their  physical  nature,  it  was 
for  a  long  time  believed  that  they  were  composed  of  a  compact 
centre,  surrounded  by  a  luminous  atmosphere.  On  this  subject  the 
opinion  of  M.  Babinet,  who  must  be  regarded  as  good  authority  on 
such  questions,  is  as  follows  :  '  Comets  cannot  exercise  any  materiaJ 
influence  upon  our  globe  ;  and  the  earth,  should  it  traverse  a  comet 
in  its  entire  breadth,  w<  uld  perceive  it  no  more  than  if  it  should  jrosw 


a  cloud  A  hundred  thousand  millions  of  tmi»s  lighter  than  our  at- 
mosphere, and  which  could  no  more  make  its  way  through  our  air 
than  the  slightest  puff  of  an  ordinary  bellows  could  make  its  way 
through  an  anvil.'  It  would  be  difficult  to  find  a  comparison  more 
reassuring. ' '  * 

What  are  the  1332'  °F  TIIE  FlXED  S'rARS'  ~ The  Fixed 
Fixed  Stars  Stars  are  all  supposed  to  be  immensely  large 
suppose*/ to  be?  bodieg;  like  our  Qwn  ^  shining  by  their 

c.wn  liglit,  which  they  dispense  to  systems  of  their  own 

„  ,  1383.  They  are  classed  by  their  apparent 

fixed  stars  magnitudes,  those  of  the  sixth  magnitude  being 
classified?  the  smallest  tnat  can  be  geeu  by  the  naked 

eye.     Stars  which  can  be  seen  only  by  means  of  the  telescope 

*  THE  COMET  OP  1853.  —  Mr.  Hind,  in  a  letter  to  the  Dmdon  Times,  give? 
the  following  particulars  with  regard  to  the  comet  which  appeared  during 
the  year  now  closing  (1853): 

"  The  comet  which  has  been  so  conspicuous  during  the  last  week  was  very 
favorably  seen  here  on  Saturday,  and  again  on  Sunday  evening.  On  the 
latter  occasion,  allowing  for  the  proximity  of  the  comet  to  the  horizon,  and 
the  strong  glow  of  twilight,  its  nucleus  was  fully  as  bright  as  an  average 
star  of  the  first  magnitude  ;  the  tail  extended  about  three  degrees  from  the 
bead.  When  viewed  in  the  comet-seeker,  the  nucleus  appeared  of  a  bright 
gold  color,  and  about  half  the  diameter  of  the  planet  Jupiter,  which  was 
shining  at  the  same  time  in  the  southern  heavens,  and  cou'd  be  readily  com- 
pared with  the  comet.  The  tail  proceeds  directly  from  the  head  in  a  single 
stream,  an]  not,  as  sometimes  remarked,  in  two  branches.  The  distance  of 
this  body  irom  the  earth  at  8  o'clock  last  evening,  was  80,000,000  miles  ; 
and  hence  it  results,  that  the  actual  diameter  of  the  bright  nucleus  was 
8000  miles,  or  about  equal  to  that  of  the  earth,  while  the  tail  had  a  real 
'ength  of  4,500,000  miles,  and  a  breadth  of  250,000,  which  is  rather  over  the 
distance  separating  the  moon  from  the  earth.  It  is  usual  to  assume  that 
the  intensity  of  a  comet's  light  varies  as  the  reciprocal  of  the  products  of 
the  squares  of  the  distance  from  the  earth  and  sun;  but  the  present  one  has 
undergone  a  far  more  rapid  increase  of  brilliancy  than  would  result  from 
this  hypothesis.  The  augmentation  of  light  will  go  on  till  the  3rd  of  Sep- 
tember, and  it  will  be  worth  while  to  look  for  the  comet  in  the  day-time 
about  that  date  ;  for  this  purpose  an  equatorially  mounted  telescope  will 
bo  required,  and  I  would  suggest  the  addition  of  a  light  green  or  red  glass, 
to  take  off  the  great  glare  of  sunlight,  the  instrument  being  adjusted  to  a 
i'ccus  on  the  planet  Venus.  This  comet  was  discovered  on  the  10th  of  June, 
by  Mr.  Kliukenfues,  of  the  Observatory  at  Gottingen,  but  was  not  bright 
enough  to  be  seen  without  a  telescope  until  about  August  13.  In  a  letter, 
copied  into  the  Times  a  few  days  since,  Sir  William  Hamilton  hints  at  the 
possibility  of  this  being  the  comet  I  had  been  expecting;  but  I  avail  myself 
oi  the  present  opportunity  of  stating  that  such  is  not  the  case,  the  elements 
of  tho  orbits  having  no  resemblance.  The  comet  referred  to  will  probably 
reappeir  between  the  years  1858  and  1861  ;  and,  if  the  perihelion  passage 
takes  place  during  the  summer  months,  we  may  expect  to  see  a  body  of  tkr 
u\-»re  imposing  aspect  than  the  one  at  present  visible." 


,W2  NATURAL    PHU 

are  caLed  telescopic  stars.     Th  >y,  also,  are  classified; 

the  classes  reaching  even  to  the  seventeenth  or  twentieth 

magnitude. 

How  many  1334.  The  number  of  the  stars  of  the 

stars  are  there  first  magnitude  is  about  twenty-four  :  of 
of  the  first  and  .,  ",  .  ,  .  ~A  »  v,  -,  .  , 

second  magni-  the  second  magnitude,  fifty  ;  of  the  third. 

tude  f  two  hundred.    The  number  of  the  smallest, 

visible  without  a  telescope,  is  from  twelve  to  fifteen 
thousand. 

How  many  of  1335.  Within  a  few  years  the  distances 
the  fixed  stars  of  nine  of  the  fixed  stars  have  been  calcu- 
have  had  their  lated>  rpj^  distance  is  so  immense,  that 
distances  very  .11.  • 

nearly  ascer-  light,  travelling  with  the  inconceivable 
tainedf  velocity  of  nearly  two  hundred  thousand 

miles  in  a  second  of  time,  from  Sirius,  is  more  than 
twenty  years  in  reaching  the  earth  ;  from  Arcturus, 
more  than  twenty-five  years  ;  and  from  the  Pole  Star, 
more  than  forty-eight  years. 

1336.  Tens  of  thousands  of  years  must  roll  away  before 
the  most  swiftly-flying  of  all  the  fixed  stars  can  complete 
even  a  small  fragment  of  its  mighty  orbit  ;  but  such  has 
been  the  advance  of  science,  that  if  a  star  move  so  slowly 
'as  to  require  five  millions  of  years  to  complete  its  revo- 
lution, its  motion  couid  be  perceived  in  one  year  ;  and  in 
ten  years  its  velocity  can  be  computed,  and  its  period  wiU 
become  known  in  the  lifetime  of  a  single  observer. 

Who  first  di  1337.  The  stars  are  the  fixed  points  k 
tided  the  stars  which  we  must  refer  in  observations  of  the 


niotions  of  a11  the  heavenly  bodies.  Hence 
the  stars  were  grouped  in  the  earliest  ages, 
(but  by  whom  we  know  not),  numbered  and  divided  into 
constellations,  the  names  of  which  have  survived  the  fal]  of 
empires. 


ASTRONOMY.  ^Scl 

What  probably  iS38.  It  is  generally  supposed  that  part, 
causes  the  dif-  ^  llot  au  of  t}ie  ciiffereilce  in  the  apparent 
fcrenceinthe  .  '  ,  .  .  *  {_  ... 

^parent  size     magnitudes  ol  the  stars  is  owing  to  the  dif- 

ofthe  stars?      ference  in  their  distance. 

1339.  The  distance  of  the  stars,  according  to  Sir  J. 
ilerschel,  cannot  be  less  than  19,200,000,000,000  miles. 
How  much  greater  it  really  is,  we  know  not,  except  in 
a  few  cases. 

1340.  Although  the  stars  generally  appear  fixed,  they  all  have 
motion ;  but  their  distance  being  so  immensely  great,  a  rapid  mo- 
tion would  not  perceptibly  change  their  relative  situation  in  two  or 
three  thousand  years.     Some  have  been  noticed  alternately  to  ap- 
pear and  disappear.     Several  that  were  mentioned  by  ancient  as- 
tronomers are  not  now  to  be  seen ;  and  some  are  now  observe*1, 
which  were  unknown  to  the  ancients. 

1841.  Many  stars  which  appear  single  to  the  naked  eye,  when 
viewed  through  powerful  telescopes,  appear  double,  treblo,  and  evei. 
quadruple.  Some  are  subject  to  variation  ir..  their  apparent  magni- 
tude, at  one  time  being  of  the  second  or  third,  and  at  another  of 
the  fifth  or  sixth  magnitude. 

What  is  the  '  1342.  The  Galaxy,  or  Milky  Way,  is  a 
Galaxy?  remarkably  light,  broad  zone,  visible  in 

the  heavens,  passing  from  noith-east  to  south-west.  It 
is  known  to  consist  of  an  immense  number  of  stars, 
which,  from  their  apparent  nearness,  cannot  be  distin- 
guished from  each  other  by  the  naked  eye. 

1343.  Sir  Wm.  Herschel  saw,  in  the  course  of  a  quarter  of  an 
hour,  the  astonishing  number  of  116,000  stars  pass  through  the 
deld  of  his  telescope,  while  it  was  directed  to  the  milky  way. 

1344.  The  ancients,  in  reducing  astronomy  to  a  sci- 
ence, formed  the  stars  into  clusters,  or  constellations,  to 
which  they  gave  particular  names. 

1345.  The  number  of  constellations  among  the  ancienta 
\vas  about  50.     The  moderns  have  added  about  50  more. 

1346.  Oar  observations  of  the  stars  and  nebulae  are  confined 
principally  to  those  of  the  northern  hemisphere.     Of  the 

tious  near  the  south  yole  we  know  but  little. 


NATUliAL    PHILOSOPHY. 

What  effect  1847.  In  determining  the  true  place  of  any 

]a$h^onrthe  of  the  celestial  bodies>  the  refractive  power  of 
length  oj  tht  the  atmosphere  must  always  be  taken  into 
**y*  consideration.  This  property  of  the  atmo- 

sphere adds  to  the  length  of  the  days,  by  causing  the  sun 
to  appear  before  it  has  actually  risen,  and  by  detaining  its 
appearance  after  it  has  actually  set. 

1348.  On  a  celrstial  globe,  the  largest  star  in  each  constellation 
is  usually  designated  by  the  first  letter  of  the  Greek  alphabet,  and 
the  next  largest  by  the  second,  &c.  When  the  Greek  alphabet  is 
exhausted,  the  English  alphabet,  and  then  numbers,  are  used. 

Wh  ar  the  1349.  The  stars,  and  other  heavenly  bodies 
stars  never  are  never  seen  in  their  true  situation,  because 

seen  in  their  ^  motion  of  light  is  progressive  ;  and.  during 
true  position  ?  .  6 

the  time  that  light  is  coming  to  the  earth, 

the  earth  is  constantly  in  motion.  In  order,  therefore,  to 
see  a  star,  the  telescope  must  be  turned  somewhat  before 
the  star,  and  in  the  direction  in  which  the  earth  moves. 

Wliat  is  meant  ^  35°*  ^ence'  a  ray  °^  ^8nt  Passing  through 
by  the  aberra-  the  centre  of  the  telescope  to  the  observer's 
Hon  of  light  *  e^Q  (joeg  not  comci(je  wjth  a  direct  line  from 

his  eye  to  the  star,  but  makes  an  angle  with  it ;  and  this  is 
termed  the  aberration  of  light. 

What  is  the  1351.  The  daily  rotation  of  the  earth  on  its 
P^lar  Star  ?  axjg  causes  the  whole  sphere  of  the  fixed 
stars,  &c.,  to  appear  to  move  round  the  earth  every  twenty- 
four  hours  from  east  to  west.  To  the  inhabitants  of  the 
northern  hemisphere,  the  immovable  point  on  which  the 
whole  seems  to  turn  is  the  Pole  Star.  To  the  inhabitants 
r/f  the  southern  hemisphere  there  is  another  and  a  cor- 
responding point  in  the  heavens. 
What  is  the  1852.  Certain  of  the  stars  surrounding  the 

dtde- of  per-     UOTt\l  pole  never  set  to  us.     These  are  in- 

petual  appar-  ,  1,7.11 

ition  and  of      eluded   hi  a  circle  parallel  with   t!;o  equator, 


ASTKONOMY.  380 


to 


^nd  in  every  part  equally  distant  from  tiro 
Citation?  north  pole  star.  This  circle  is  called  the 
circle  of  perpetual  apparition.  Others  never  rise  to  us. 
These  are  included  in  a  circle  equally  distant  from  the 
south  pole  ;  and  this  is  called  the  circle  of  perpetual  oo- 
cullation.  Some  of  the  constellations  of  the  southern 
hemisphere  are  represented  as  inimitably  beautiful,  par- 
ticularly the  cross. 

What  is  par-      1353.  The  parallax  of  a  heavenly  body 
allax*  ig  the  angular  distance  between  the  true 

and  the  apparent  situation  of  the  body. 
Describe          1354.  la  Fig.  196,   A  G  B   represents  the  earth 
f  \g.  196.   ami  (j  the  moon.     To  a  spectator  at  A,  the  moon 


i 

would  appear  at  F;  while  to  another,  at  B,  the  moon  woul;! 
appear  at  D  ;  but,  to  a  third  spectator,  at  G,  the  centre  of  the 
earth,  the  moon  would  appear  at  E,  which  is  the  true  situation, 
The  distance  from  F  to  E  is  the  parallax  of  the  moon  when 
vicvTed  from  A,  and  the  distance  from  E  to  D  is  the  parallax 
when  viewed  from  B. 

1355.  From  this  it  appears  that  the  situation  of  thf  heavenly 
bodies  must  always  be  calculated  from  the  centre  of  the  earth  ; 
and  the  observer  must  always  know  the  distance  between  the 
place  of  his  observation  and  the  centre  of  the  earth,  in  order  tc 
make  the  necessary  calculations  to  determine  the  true  situation 
of  the  body.  Allowance,  also,  must  be  made  for  the  refraction 
jl  the  atmosphere. 


<*6fc  NATURAL   PHILOSOPHY. 

lions  of  miles  every  year.  This  will  give  him,  at  the  same  time, 
*:  motion  of  more  than  68  000  miles  in  an  hour  in  a  different 
direction.  If  the  question  be  asked,  why  each  individual  is  not 
sensible  of  these  tremendously  rapid  motions,  the  answer  is> 
that  no  one  ever  knew  what  it  is  to  be  without  them.  We  can- 
not be  sensible  that  we  have  moved  without  feeling  our  motion, 
as  when  in  a  boat  a  current  takes  us  in  one  direction,  while  a 
gentle  wind  carries  us,  at  the  same  time,  in  another  direction. 
It  is  only  when  our  progress  is  arrested  by  obstacles  of  some 
kind  that  we  can  perceive  the  difference  between  a  state  oi 
motion  and  a  state  of  rest. 

What  would          1297.  The  rapid  motion  of  a  thousand  miles  in 

le  the  conse-  an  hour  is  not  sufficient  to  overcome  the  centri- 

yutnce  if  the  petai  force  cause(i  by  gravity  ;  but,  if  the  earth 

sarth  should  r  J  \ ? 

revolve  on  its  should  revolve  around  its  axis  seventeen  times  in 

axis  once  in       a  day,  instead  of  once,  all  bodies  at  the  equator 
would  be  lifted  up,  and  the  attraction  of  gravita- 
tion would  be  counterbalanced,  if  not  wholly  overcome. 

1298.  Certain  irregularities  in  the  orbit  of  the  earth  have 
been  noticed  by  astronomers,  which  show  that  it  is  deviating 
from  its  elliptical  form,  and  approaching  that  of  a  circle.  In 
this  fact,  it  has  been  thought,  might  be  seen  the  seeds  of  decay. 
But  Laplace  has  demonstrated  that  these  irregularities  proceed 
from  causes  which,  in  the  lapse  of  immensely  long  periods, 
counterbalance  each  other,  and  give  the  assurance  that  there  is 
no  other  limit  to  the  present  order  of  the  universe,  than  the  will 
of  its  great  Creator. 

Describe  the  1299.  OF  MARS.  —  Next  to  the  earth  is 
flatiet  Mars,  the  planet  Mars.  It  is  conspicuous  for  its 
fiery-red  appearance,  which  is  supposed  by  Sir  Johi) 
llersclier*  to  be  caused  by  the  color  of  its  soil. 

*  Sir  John  Herschel  is  the  son  of  Sir  "William  Herschel,  the  discov^rr 
oi'  *he  piauet  Uranu». 


ASTRONOMY.  36? 

The  degree  of  heat  and  light  at  Mars  is  less  than  half  of 
that  received  by  the  earth. 

1300.  OP  THE  MINOR  PLANETS.— It. has  already  been  mention 
ed  that  between  the  orbits  of  Mars  and  Jupiter  one  hundred  and 
thirteen  small  bodies  have  been  discovered,  which  are  called  the 
minor  planets.  It  is  a  remarkable  fact,  that  before  the  discovery 
of  Bode's  law  (see  No.  1232)  certain  irregularities  observed  in  the 
motions  of  the  old  planets  induced  some  astronomers  to  sup- 
pose that  a  planet  existed  between  the  orbits  of  Mars  aLd  Jupi- 
tei.  The  opinion  has  been  advanced  that  these  small  bodies 
originally  composed  one  larger  one,  which,  by  some  unknown 
force  or  convulsion,  burst  asunder.  This  opinion  is  maintained 
with  much  ingenuity  and  plausibility  by  Sir  David  Brewster. 
(See  Ed'm.  Encyc.,  art.  ASTRONOMY.)  Dr.  Brewster  further 
supposes  that  the  bursting  of  this  planet  may  have  occasioned 
"the  phenomena  of  meteoric  atones ;  that  is,  stones  which  have 
fallen  on  the  earth  from  the  atmosphere. 

Describe  the  1301.  OF  JUPITER.  — Jupiter  is  the  largest 
planet  Jupiter.  pianet  Of  tm3  soiar  gjstem,  and  the  most  bril- 
liant, except  Venus.  The  heat  and  light  at  Jupiter  art 
about  twenty-five  times  less  than  that  at  the  earth.  This 
planet  is  attended  by  four  moons,  or  satellites,  the  shadows 
of  some  of  which  are  occasionally  visible  upon  his  surface. 

1302.  The  distance  of -those  satellites  from  the  planet  are 
two,  four,  six  and  twelve  hundred  thousand  miles,  nearly. 

The  nearest  revolves  around  the  planet  in  less  than  two  days ; 
the  next,  in  less  than  four  days ;  the  third,  in  less  than  eight 
days ;  and  the  fourth,  in  about  sixteen  days. 

These  four  moons  must  aiford  considerable  light  to  the  inhab- 
itants of  the  planet ;  for  the  nearest  appears  to  them  four  times 
the  size  of  our  moon,  the  second  about  the  same  size,  the  third 
somewhat  less,  and  the  fourth  about  one-third  the  diameter  of 
our  moon. 


388 


NATURAL    PHILOSOPHY. 


at  another  time,  the  whole  surface  appearing  resplendent, 
This  is  caused  by  the  relative  position  of  the  moon  with 
regard  to  the  sun  and  the  earth.  The  moon  is  an  opaque 
body,  and  shines  only  by  the  light  of  the  sun.  When, 
therefore,  the  moon  is  between  the  earth  and  the  sun,  it 
presents  its  dark  side  to  the  earth ;  while  the  side  presented 
to  the  sun,  and  on  which  the  sun  shines,  is  invisible  to  the 
earth.  But  when  the  earth  is  between  the  sun  and  the 
moon,  the  illuminated  side  of  the  moon  is  visible  at  the 
earth. 

'Describe  1364.  In  Fig.  197,  let  S  be  the  sun,  E  the  earth, 
Fig,  197.  and  A  B  C  D  the  moon  in  different  parts  of  hei 

Via,    1*7 


orbit  When  the  moon  is  at  A,  its  dark  side  will  be  towards 
the  earth,  its  illuwvtated  part  being  always  towards  the  sun. 
Hence  the  moon  will  appear  to  us  as  represented  at  a.  But 
when  it  has  advanced  in  its  orbit  to  B,  a  small  part  of  its 
illuminated  fide  coming  in  sight,  it  appears  as  represented  at  b. 
and  is  said  to  be  horned.  When  it  arrives  at  C,  one-half  its 
illuminated  side  is  visible,  and  it  appears  as  at  c.  At  C,  and 
in  the  opposite  point  of  its  orbit,  the  moon  is  said  to  be  in  quad' 
At  D  its  appearance  is  as  represented  at  d,  and  it  is 
to  be  gibboits.  At  E  all  the  illuminated  side  is  towards 
•.is,  aud  wo  have  a  full  moon.  During  the  other  half  of  it* 


ABi'KO^OMY. 

revolution,  less  and  less  of  its  illuminated  side  is  seen,  till  it 
again  becomes  invisible  at  A. 

Wfiat  is  (he  1365.  The  mean  difference  in  the  rising  of  the 

mean  differ-  moon,  caused  by  its  daily  motion,  is  a  little  lusr 
ruing  of  the  ^an  an  ^our-  But,  on  account  of  the  different 
moon  from  day  angles  formed  with  the  horizon  by  different  parts 
to  day .  Qf  tne  eciiptiC)  it  happens  that  for  six  or  eight 

nights  near  the  full  moons  of  September  and  October  the  moon 
rises  nearly  as  soon  as  the  sun  is  set.  As  this  is  a  great  con- 
venience to  the  husbandman  and  the  hunter,  in- 
^the  l Harvest  asmuca  as  li  affords  tnein  light  to  continue  their 
and  the  Hunt-  occupation,  and,  as  it  were,  lengthens  out  thei/ 
er'sMoon,  and  day,  the  first  is  called  the  harvest  moon,  and  the 
occur  t  second  the  hunter's  moon.  These  moons  are 

always  most  beneficial  when  the  moon's  ascending 
node  is  in  or  near  Aries. 

1366.  The  following  signs  are  used  in  our  common  almanacs 
to  denote  tl»e  different  positions  and  phases  of  the  moon.      }  or 
})   denote  the  moon  in  the  first  quadrature,  that  is,  the  quad- 
rature between  change  and  full ;   C  or   <£  denotes  the  moon  in 
the  last  quadrature,  that  is,  the  quadrature  between  full  and 
cnange.      $  denotes  new  moon  ;  Q  denotes  full  moon. 

1367.  When  viewed  through  a  telescope,  the  surface  of  the 
moon  appears  wonderfully  diversified.     Large  dark  spots,  sup- 
posed to  be  excavations,  or  valleys,  are  visible  to  the  eye; 
some  parts  also  appear  more  lucid  than  the  general  surface 
These  are  ascertained  to  be  mountains,  by  the  shadows  whicV- 
they  cast.     Maps  of  the  moon's  surface  have  been  drawn,  on 
which  most  of  these  valleys  and  mountains  are  delineated,  and 
names   are   given   to   them.     Some  of  these    excavations   are 
thought  to  be  four  miles  deep,  and  forty  wide.     A  high  ridge 
generally  surrounds  them,  and  often  a  mountain  rises  in  the 
centre,     These  immense  depressions  probably  very  much  re- 
semble what  would  be  the  appearance  of  the  earth  at  the  m  >OP 


390 


NATURAL    PHILOSOPHY. 


were  all  the  seas  and  lakes  dried  up.  Some  of  the  mountain* 
are  supposed  to  be  volcanic. 

What  are  the  1368.  OF  THE  TIDES. — The  tides  are  the 
Tides  f  regular  rising  and  falling  of  the  water  of 

the  ocean  twice  in  about  twenty-five  hours.  They  are 
occasioned  by  the  attraction  of  the  moon  upon  the 
matter  of  the  earth ;  and  they  are  also  affected  by  that 
of  the  sun. 

Explain  1369.  Let  M,  Fig.  198,  be  the  moon  revolving  in 
Fig.  198.  her  orbit ;  E,  the  earth  covered  with  water ;  and  S, 

Fig.  196. 


the  sun.  Now,  the  point  of  the  earth's  surface,  which  is  nearest 
to  the  moon,  will  gravitate  towards  it  more,  and  the  remoter 
point  less,  than  the  centre,  inversely  as  the  squares  of  their  re- 
spective distances.  The  point  A,  therefore,  tends  away  from 
the  centre,  and  the  centre  tends  away  from  the  point  B ;  and  in 
each  case  the  fluid  surface  must  rise,  and  in  nearly  the  same 
degree  in  both  cases.  The  effect  must  be  diminished  in  propoi- 
tion  to  the  distance  from  these  points  in  any  direction ;  and  at 
the  points  C  and  D,  ninety  degrees  distant,  it  ceases.  But 
there  the  level  of  the  waters  must  be  lowered,  because  of  the 
exhaustion  at  those  places,  caused  by  the  overflow  elsewhere. 
Thus  the  action  of  the  moon  causes  the  ocean  to  assume 
the  form  of  a  spheroid  elongating  it  in  the  direction  of  the 

IDOOD. 


ASTEOKOMT.  391 

Thus  any  particular  place,  as  A,  while  passing  from  under  the 
moon  till  it  comes  under  the  moon  again,  has  two  tides.  But 
the  moon  is  constantly  advancing  in  its  orbit,  so  that  the  earth 
must  a  little  more  than  complete  its  rotation  before  the  place  A 
comes  under  the  moon.  This  causes  high  water  at  any  place 
about  fifty  minutes  later  each  successive  day. 

As  the  moon's  orbit  varies  but  little  from  the  ecliptic, 
the  moon  is  never  more  than  29°  from  the  equator,  and  is 
generally  much  less.  Hence  the  waters  about  the  equator, 
being  nearer  the  moon,  are  more  strongly  attracted,  and 
fche  tides  are  higher  than  towards  the  poles. 

1370.  The  sun  attracts  the  waters  as  well  as  the  moon.  When 
the  moon  is  at  full  or  change,  being  in  the  same  line  of  direction 
(see  Fig.  198),  the  sun  acts  with  it ;  that  is,  the  sun  and  moon 
tend  to  raise  the  tides  at  the  same  place,  as  seen  in  the  figure. 
The  tides  are  then  very  high,  and  are  called  spring  tides. 
Explain  But  when  the  moon  is  in  its  quarters,  as  in  Fig.  199, 
Fig.  199.  the  sun  and  moon  being  in  lines  at  right  angles,  tend 

Pig.  199. 


to  raise  tides  at  different  places — namely,  the  moon  at  C  and  D, 
and  the  sun  at  A  and  B.  Tides  that  are  produced  when  the 
moon  is  in  its  quarters  are  low,  and  are  called  neap  tides. 

1371.  There  are  so  many  natural  difficulties  to  the  free  prog- 
ress of  the  tides,  that  the  theory  by  which  they  are ,  accounted 
for  is,  in  fact,  and  necessarily,  the  most  imperfect  of  all  the 
theories  connected  with  astronomy.  It  is,  however,  indisputable 
that  the  moon  has  an  effect  upon  the  tides,  although  it  be  not 


392  NATURAL    PHILOSOPHY. 

equally  felt  in  all  places,  owing  to  the  indentations  of  the  coast, 
the  obtructions  of  islands,  continents,  &c.,  which  prevent  the 
free  motion  of  the  waters.  In  narrow  rivers  the  tides  are  fre- 
quently very  high  and  sudden,  from  the  resistance  afforded  by 
their  banks  to  the  free  ingress  of  the  water,  whence  what  would 
otherwise  be  a  tide,  becomes  an  accumulation.  It  has  been  con- 
stantly observed,  that  the  spring  tides  happen  at  the  new  and 
full  moon,  and  the  neap  tides  at  the  quarters.  This  circum 
stance  is  sufficient  in  itself  to  prove  the  connexion  between  the 
influence  of  the  moon  and  the  tides. 

1372.  An   Eclipse  is  a  total  or  partial  ob- 
What  is  an  ,.          ,,          ,  1111,1- 

Eclipse  ?        scuration  of  one  heavenly  body  by  the  interven- 
tion of  another. 

The  situation  of  the  earth  with  regard  to  the 
When   does  an  ,1^.1  .  ,  -, 

eclipse    of    the    moon»  or  rather  of  the  moon  with  regard  to  the 

sun  or  of  the  earth,  occasions  eclipses  both  of  the  sun  and 
moon  take  place?  moon<  Thoge  of  fhe  gun  take  p]ace  wheQ  ^ 

moon,  passing  between  the  sun  and  earth,  intercepts  his  rays. 
Those  of  the  moon  take  place  when  the  earth,  coming  between 
the  sun  and  moon,  deprives  the  moon  of  his  light.  Hence,  an 
eclipse  of  the  sun  can  take  place  only  when  the  moon  changes, 
and  an  eclipse  of  the  moon  only  when  the  moon  fulls ;  for,  at 
the,  time  of  an  eclipse,  either  of  the  sun  or  the  moon,  the  sun 
earth,  and  moon,  must  be  in  the  same  straight  line. 

If  the  moon  revolved  around  the  earth  in  the 
Why    is    there  .         .        ,  .  ,     ,  , 

not  an  eclipse  at    samc  plane  m  which  the  earth  revolves  around 

every  new  and  the  sun,  that  is,  in  the  ecliptic,  it  is  plain  that 
full  moon?  ^  gun  woui(j  be  eoiipseci  at  every  new  moon, 

anl  the  moon  would  be  eclipsed  at  every  full.  For,  at  each  of 
these  times,  these  three  bodies  would  be  in  the  same  stiaight 
line.  But  the  moon's  orbit  does  not  coincide  with  the  ecliptic, 
but  is  inclined  to  it  at  an  angle  of  about  5"  20'.  Hence,  since 
the  apparent  diameter  of  the  sun  is  but  about  £  a  degree,  and 
that  of  the  moon  about  the  same,  n }  eclipse  will  take  place  at 


ASTKONOMT.  393 

new  or  full  moon,  unless  the  moon  be  within  ^  a  deg/ee  of  the 
ecliptic,  that  is,  in  or  near  one  of  its  nodes.  It  is  found  tbit 
if  the  moon  be  within  16£°  of  a  node  at  time  of  change,  it  will 
be  so  near  the  ecliptic,  that  the  sun  will  be  more  or  le&a 
eclipsed ;  if  within  12°  at  time  of  full,  the  moon  will  be  more 
or  less  eclipsed. 

Why  are  there  1373.  It  is  obvious  that  the  moon  will  be 
more  eclipses  of  oftener  within  16^°  at  the  time  of  new  moon, 

^moo^ini  than  withiu  12°  at  the  time  of  ful1;  cons« 
given  course  of  quently,  there  will  be  more  eclipses  of  the  sun 
years?  than  of  the  moon  in  a  course  of  years.  As  the 

nodes  commonly  come  between  the  sun  and  earth  but  twice  in 
a  year,  and  the  moon's  orbit  contains  360°,  of  which  16^-°,  the 
limit  of  solar  eclipses,  and  12°,  the  limit  of  lunar  eclipses,  are 
but  small  portions,  it  is  plain  there  must  -be  many  new  and  full 
moons  without  any  eclipses. 

Although  there  are  more  eclipses  of  the  sun 

E*Plain  Ft8-       than    of  the   moon,  yet   more  eclipses   of  the 

inoon  will  be  visible  at  a  particular  place,  as 

Boston,  in  a  course  of  years,  than  of  the  sun.     Since  the  sun  is 

very  much  larger  than  either  the  earth  or  moon,  the  shadow  of 

Fig.  200. 


these  bodies  must  always  terminate  in  a  point ;  that  is,  it  must 
always  be  a  cone.  In  Fig.  200,  let  S  be  the  sun,  m  the  moon, 
and  E  the  earth.  The  sun  constantly  illuminates  half  the  earth's 
surface,  that  is,  a  hemisphere ;  and  consequently  it  is  visible  to 
a')  in  this  hemisphere.  But  the  moon's  shadow  falls  upon  a 
part  only  of  this  hemisphere ;  and  hence  the  sun  appears 
eclipsed  to  a  part  only  of  those  to  whoa  it  is  visible,  Some- 
times, when  the  moon  is  at  its  greatest  distance,  its  shadow,  0 


394  NATURAL    PHILOSOPHY. 

m.,  terminates  before  it  reaches  the  earth.  In  eclipses  of  this 
kind,  to  an  inhabitant  directly  under  the  point  0,  the  outermo*'. 
edge  of  the  sun's  disc  is  seen,  forming  a  bright  ring  around  the 
moon ;  from  which  circumstance  these  eclipses  are  called  annu- 
lar, from  anrndus,  a  Latin  word  for  ring. 

Besides  the  dark  shadow  of  the  moon,  m  O,  in  which  all  the 
light  of  the  sun  is  intercepted  (in  which  case  the  eclipse  is 
called  total],  there  is  another  shadow,  r  C  D  S,  distinct  from 
the  former,  which  is  called  the  penumbra.  Within  this,  only  a 
part  of  the  sun's  rays  are  intercepted,  and  the  eclipse  is  called 
partial.  If  a  person  could  pass,  during  an  eclipse  of  the  sun, 
from  0  to  D,  immediately  on  emerging  from  the  dark  shadow, 
0  m,  he  would  see  a  small  part  of  the  sun ;  and  would  con- 
tinually see  more  and  more  till  he  arrived  at  D,  where  all 
shadow  would  cease,  and  the  whole  sun's  disc  be  visible.  Ap- 
pearances would  be  similar  if  he  went  from  0  to  C.  Hence 
the  penumbra  is  less  and  less  dark  (because  a  less  portion  of 
the  sun  is  eclipsed),  in  proportion  as  the  spectator  is  more  re« 
mote  from  0,  and  nearer  G  or  D.  Though  the  penumbra  be 
continually  increasing  in  diameter,  according  to  its  length,  01 
the  distance  of  the  moon  from  the  earth,  still,  under  the  most 
favorable  circumstances,  it  falls  on  but  about  half  of  the  illu« 
minated  hemisphere  of  the  earth.  Hence,  by  half  the  inhab 
tants  on  this  hemisphere,  no  eclipse  will  be  seen. 

1374.  Fig.  201  represents  an  eclipse  of  th« 
Explain  Fig.       mooilt     The  jnstant  tlie  moon  enters  the  earth's 

shadow  at  x,  it  is  deprived  of  the  sun's  light 

Fig.  201. 


A.Sl'JtiUNOMY.  Ji» 

and  is  eclipsed  to  all  in  the  un illuminated  hemisphere  of  the 
earth.  Hence,  eclipses  of  the  moon  are  visible  to  at  least  twice 
as  many  inhabitants  as  those  of  the  sun  can  be ;  generally  the 
proportion  is  much  greater.  Thus,  the  inhabitants  at  a  par- 
ticular placo,  as  Boston,  see  more  eclipses  of  the  moon  than  of 
the  SUD, 

The  reason  why  a  lunar  eclipse  is  visible  to  all  to  whom 
the  moon  at  the  time  is  visible,  and  a  solar  one  is  not  so  to  all  to 
whom  the  sun  at  the  time  is  visible,  may  be  seen  from  tho 
nature  of  these  eclipses.  We  speak  of  the  sun's  being  eclipsed ; 
but,  properly,  it  is  the  earth  which  is  eclipsed.  No  change 
takes  place  in  the  sun ;  if  there  were,  it  would  be  seen  by  all 
to  whom  the  sun  is  visible.  The  sun  continues  to  diffuse  its 
beams  as  freely  and  uniformly  at  such  times  as  at  others.  .But 
these  beams  are  intercepted,  and  the  earth  is  eclipsed  only 
where  the  moon's  shadow  falls,  that  is,  on  only  a  part  of  a 
hemisphere.  In  eclipses  of  the  moon,  that  body  ceases  to 
receive  light  from  the  sun,  and,  consequently,  ceases  to  reflect 
it  to  the  earth.  The  moon  undergoes  a  change  in  its  appear- 
ance ;  and,  consequently,  this  change  is  visible  at  the  same  time 
to  all  to  whom  the  moon  is  visible ;  that  is,  to  a  whole  hemis- 
phere of  the  earth. 

1375.  The  earth's  shadow  (like  that  of  the  moon)  is  encom- 
passed by  a  per^jaDra,  C  R  S  D,  which  is  faint  at  the  edges 
towards  R  and  S,  but  becomes  darker  towards  F  and  Gr.  The 
shadow  of  the  earth  is  but  little  darker  than  the  region  of  the 
penumbra  next  to  it.  Hence  it  is  very  difficult  to  determine 
the  exact  time  when  the  moon  passes  from  the  penumbra  into 
the  shadow,  and  from  the  shadow  into  the  penumbra ;  that  is, 
when  the  eclipse  begins  and  ends.  But  the  beginning  and  end- 
ing of  a  solar  eclipse  may  be  determined  almost  instantaneously. 
1376.  The  diameters  of  the  sun  and  moon 

KgJ*  ^ap-  are  suPP°sed  to  be  divided  into  twelve  «P*] 
plied  to  'eclipses  parts,  called  digits.  These  bodies  are  said  tc 

of  the  sun  and  have  as  many  digits  eclipsed  as  there  are  of 
H'ttenuion? 

those  parts  involved  m  darkness 


NAIUKAL   PiiILO3OPHl. 

1377.  There  must  be  an  eclipse  of  the  sun  fk  sften,  al  least, 
as  the  moon,  being  near  one  of  its  nodes,  comes  between  the 
aun  and  the  earth. 

The  greatest  number  of  both  solar  and  lunar  eclipses  that  can 
take  place  during  the  year  is  seven.  The  usual  number  is  four 
two  solar  and  two  lunar. 

1378.  A  total  eclipse  of  the  sun  is  a  very  remarkable  phe 
nomenon. 

June  16,  1806,  a  very  remarkable  total  eclipse  took  place  at 
Boston.  The  day  was  clear,  and  nothing  occurred  to  prevent  accu- 
rate observation  of  this  interesting  phenomenon.  Several  stars  were 
visible  ;  the  birds  were  greatly  agitated  ;  a  gloom  spread  over  the 
landscape,  and  an  indescribable  sensation  of  fear  or  dread  pervaded 
the  breasts  of  those  who  gave  themselves  up  to  the  simple  effects  of  the 
phenomenon,  without  having  their  attention  diverted  by  efforts  of 
observation.  The  tiret  gleam  of  light,  contrasted  with  the  previous 
darkness,  seemed  like  the  usual  meridian  day,  and  gave  indescribable 
life  and  joy  to  the  whole  creation.  A  total  eclipse  of  the  sun  can 
last  but  little  more  than  three  minutes.  An  annular  eclipse  of  the 
sun  is  still  more  rare  than  a  total  one. 

1379.  OF  TIME.  —  When  time  is  calcu- 

ference  'between  ^ate(^  *>y  tne  8un'  'lt  is  calle(l  solar  time?  an(^ 
the  solar  and  the  the  year  a  solar  year  ;  but  when  it  is  calcu- 
ndereal  ear  ?  gidereal 


and  the  year  a  sidereal  year.     The  sidereal  year  is  20  min- 
utes and  24  seconds  longer  than  the  solar  ^  :ar. 

1380.  The  solar  year  consists  of  365 
days,  5  hours,  48  minutes,  and  48  seconds; 
sidereal  but  our  common  reckoning  gives  365  days 
by  only  to  the  year.  As  the  difference  amounts 
to  nearly  a  quarter  of  a  day  every  year,  it 
is  usual  every  fourth  year  to  add  a  day.  Every  fourth 
year  the  Romans  reckoned  the  6th  of  the  calends  of 
March,  and  the  following  day  as  one  day  ;  which,  on 
that  account,  they  called  bissextile,  or  twice  the  6th  day  ; 
whence  ^e  derive  the  name  of  bissextile  for  the  leap  v^>. 


ASTKONOMY.  b97 

in  which  we  give  to  February,  for  the  same  reason,  29 
days  every  fourth  year. 

1381.  A  solar  year  is  measured  from  the  time  the  earth 
sets  out  from  a  particular  point  in  the  ecliptic,  as  an  equi- 
nox, or  solstice,  until  it  returns  to  the  same  point  again. 
A  sidereal  year  is  measured  by  the  time  that  the  earth 
takes  in  making  an  entire  revolution  in  its  orbit;  or,  in 
other  words,  from  the  time  that  the  sun  takes  to  return  into 
conjuction  with  any  fixed  star. 

What  is  the  pre-  1382-  EverJ  equinox  occurs  at  a  point, 
cession  cj  the  50"  of  a  deg.  of  the  great  circle,  preceding 
the  place  of  the  equinox,  12  months  before  j 
and  this  is  called  the  precession  of  the  equinoxes.  It  is 
this  circumstance  which  has  caused  the  change  in  the  situ- 
ation of  the  signs  of  the  zodiac,  of  which  mention  has 
already  been  made. 

1383.  The  earth's  diurnal  motion  on  an  inclined  axis, 
together  with  its  annual  revolution  in  an  elliptic  orbit, 
occasions  so  much  complication  in  its  motion  as  to  pro- 
duce many  irregularities;  therefore,  true  equal  time 
cannot  be  measured  by  the  sun.  A  clock  which  is 
always  perfectly  correct  will,  in  some  parts  of  the  year, 
be  before  the  sun,  and  in  other  parts  after  it.  There  are 
When  do  the  ^ut  *°ur  Peri°ds  m  which  the  sun  and  a 
sun  and  dock  perfect  clock  will  agree.  These  are  the 
agree?  15th  of  ^prf],  the  15th  of  June,  the  1st  oi 

September,  and  the  24th  of  December. 

What  is  the  1384.  The  greatest  difference  between 
greatest  dif-  true  and  apparent  time  amounts  to  between 
sixteen  and  seventeen  minutes.  Tables  of 


apparent   equation  are  constructed  for  the  purpose  of 
*  f  pointing  oat  and  correcting  these  differences 


398  NATURAL    PHILOSOPHY. 

between  solar  time  and  equal  or  mean  time,  tne  denomina- 
tion given  by  astronomers  to  true  time. 

1385.  As  it  may  be  interesting  to  those  who  have  access  to  a 
celestial  globe  to  know  how  to  find  any  particular  star  or  con- 
stellation, the  following  directions  are  subjoined. 

There  is  always  to  be  seen,  on  a  clear  night,  a  beautiful  clus- 
ter  of  seven  brilliant  stars,  which  belong  to  the  constellation 
"  Ursa  Major,"  or  the  Great  Bear.  Some  have  supposed  that 
they  will  aptly  represent  a  plough ;  others  say  that  they  are 
more  like  a  wagon  and  horses,  the  four  stars  representing  the 
body  of  the  wagon,  and  the  other  three  the  horses.  Hence 
they  are  called  by  some  the  plough,  and  by  others  they  are 
called  Charles1  wain,  or  wagon. 

Fig.  202  represents  these  seven  stars ;  KB-  aoa- 

«  b  a  g  represent  the  four,  and  e  z  B  \ 

the   other   three   stars.      Perhaps   they 
may   more  properly  be   called  a  large  F  \\ 

dippei    of  which   e  z  B   represent  the  j   \ 

handle.     If  a  line  be  drawn  through  the  I       '*  .  a 

stars  I  *nd  a,  and  carried  upwards,  it  (  j 

will  pass  a  little  to  the  left,  and  nearly         +/z    e  i  + 
touch  a  star  represented  in  the  figure  by          /  g 

P.     This  is  the  polar  star,  or  the  north 
pole  star ;  and  the  stars  b  and  a,  which   4* 
appear   to  point  to  it,  are  called  the  pointers,  because  they 
appear  to  point  to  the  polar  star. 

The  polar  star  shines  with  &  steady  and  rather  dead  kind  ol 
light.  It  always  appears  in  the  same  position,  and  the  north 
pole  of  the  earth  always  points  to  it  at  all  seasons  of  the  year. 
The  other  stars  seem  to  move  round  it  as  a  centre.  As  this  star 
is  always  in  the  north,  the  cardinal  points  may  at  any  time  be 
found  by  starlight. 

By  these  stars  we  can  also  find  any  other  star  or  constella- 
tion. 

Thus,  if  we  conceive  a  line  drawn  from  the  star  z,  leaving  B 


ASTKONOMT.  399 

a  little  to  the  left,  it  will  pass  through  the  very  brilliant  star  A. 
By  looking  on  a  celestial  globe  for  the  star  2,  and  supposing  the 
line  drawn  on  the  globe,  as  we  conceive  it  done  on  the  heavens, 
we  shall  find  the  star  and  its  name,  which  is  Arcturus. 

Conceiving  another  line  drawn  through  g  and  &,  and  extended 
some  distance  to  the  right,  it  wiU  pass  just  above  another  very 
brilliant  star.  On  referring  to  the  glcue,  we  find  it  to  be  Capella, 
or  the  goat. 

In  this  manner  the  student  m.ay  "W~«n->.  acquainted  with  th« 
appearance  of  the  whole  heavens. 


400 


NATURAL   PHILOSOPHY. 


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i  lOPirnr^iMi^oo^! 


APPENDIX. 


1386.  MECHANICS.  A  clear  understanding  of  the  prin- 
ciples of  mechanics  as  at  present  treated,  requires  a  clear 
conception  of  the  terms  Force,  Energy,  Power,  and  Work. 

1387.  Force  is  any  agency  which  produces. 
Define  Force.  *    • 

or  tends  to  produce,  or  arrest,  motion  of 

matter. 

Mention  the  1388.  The  various  forces  in  nature  are  the 

principal  attractions  of  Gravitation,  Cohesion,  Adhe- 
sion, Electricity,  Magnetism,  and  Chemical 
Affinity;  also  the  repellent  action  manifested  among  the 
molecules  of  a  body  when  the  mass  is  compressed,  and  the 
repulsions  exhibited  between  bodies  under  the  effects  of 
Electricity  and  Magnetism.  To  these  should  also  be  added 
the  specific  force  or  forces  that  produce  motion  in  living 
things.  The  definition  of  force  will  also  include  any 
matter  in  motion,  because  a  moving  body  striking  any 
other  body  will  produce  or  arrest  motion. 

1389.   Work  is  force    exerted  through  a 
Define  Work. 

certain  distance. 

It  is  highly  important  in  Mechanics  that  we  should  be 
able  to  compare  the  efficiency  of  different  forces,  or  their 
ability  to  produce  motion  in  bodies.  This  requires  the 
adoption  of  some  kind  of  standard  measure.  English  and 
American  authors  have  adopted  one  which  is  called  the 
"Unit  of  Work." 

17* 


404  MECHANICS. 

What  is  a  1390.  A  single  Unit  of  Work  is  a  force 

Unit  of  Work  ?    Of  one  pound  exerted  through   a  distance 
of  one  foot. 

rr     .  1391.  The  amount  of  work  performed  by 

JJ.QW  is  any 

amount  of  work  any  force  acting  through  any  distance  is  ex- 
expri  pressed  by  the  product  of  the  force  in  pounds 

multiplied  by  the  distance  in  feet.  Thus  when  a  weight 
of  10  pounds  is  raised  through  a  height  of  3  feet,  30  units 
of  work  are  performed. 

EXAMPLES   FOR   SOLUTION. 

1392.  (1)  How  many  units  of  work  are  performed  when 
a  block  of  stone  weighing  2000  Ibs.  is  raised  to  a  height 
of  15  feet  ?  Ans.  30,000. 

(2)  A  car  weighing  one  ton  requires  a  steady  pull  of 
8  Ibs.  to  keep  it  in  motion  on  a  level  track,  How  many 
units  of  work  are  performed  in  hauling  the  car  one  mile 
(5280  ft.)  ?  Ans.  42,240. 

(3)  -How  many  units  of  work  are  performed  by  a  stream 
that  discharges  100  cubic  feet  of  water  per  minute  over  a 
fall  of  12  ft.  ?     (A  cubic  foot  of  water  weighs  62^  pounds.) 

Ans.  75,000  units  per  minute. 

What  term  is  1393'  Instea(i  of  Units  of  Work,  the  term 
used  iistead  of  «  foot  pounds"  is  used  by  many  writers. 

Units  of  Work?       mu    -n        i         -j.  /•  *         i     • 

The  French  unit  for  measure  of  work  is 

that  <xf  a  kilogram  exerted  through  the  distance  of  a  metre, 
and  ifl  called  a  kilogram-metre. 

1394.  The  term  Energy  signifies  ability  to 
perform  work.*     Thus  if  a  hundred -pound 
weight  be  lifted  to   a  height  of  ten  feet,  it  is  rendered 

*  Elementary  Physics.    By  Balfour  Stewart. 


MECHANICS.  405 

capable  of  performing  1000  units  of  work;   this,  there- 
fore, is  its  Energy. 

now  does  1395>  The  same  weight  lying  upon  the 

Energy  differ      ground,  although  it  exerts  a  pressure  of  100 
from  pressure?  ,,       ,  _,  n 

IDS.,  has  no  Energy,  because,  until  it  is  lifted, 

it  has  no  power  to  perform  work. 

1396.  Power  in  Mechanics  signifies  ability 

Define  Power.  J 

to  perform  a  certain  amount  of  work  in  a 

given  time. 

The  most  common  unit  for  measurement  of  power  is  the 
Horse  Power.  It  is  employed  in  comparing  the  efficiency 
of  engines,  water-wheels,  and  other  motors. 

Define  Horse     1397.  An  engine  of  one-horse-power  is  capa- 
Power.  big  Of  raising  33?000  pounds  one  foot  high  in 

one  minute,  or,  in  other  words,  of  performing  33,000  units 
of  work  in  a  minute. 

An  engine  which  can  perform  twice  as  much  work  in  a 
minute,  or  the  same  amount  in  hah01  a  minute,  is  a  two- 
horse-power  engine. 

1398.  This  unit  of  measure  for  engines  was  first  employed  by 
James  Watt  when  steam-engines  were  first  employed  in  the  place 
of  horses  in  working  pumps.  It  was  then  supposed  that  this  was 
an  average  of  the  power  of  working-horses.  It  is  now  known  to  be 
much  too  high. 

A  careful  series  of  experiments  by  better  methods  has  made 
known  the  fact  that  two-thirds  of  the  above  amount,  or  22,000  units 
of  work  in  a  minute,  is  more  nearly  the  power  of  a  horse  of  average 
strength ;  but  the  term,  when  applied  to  motors,  implies  the  amount 
as  first  estimated  in  the  time  of  Watt. 

The  strength  of  a  man  being  much  less  than  that  of  a  horse, 
'his  power  to  perform  work  is  of  course  proportionately  less.  The 
power  of  a  man  varies  considerably  with  the  method  by  which  he 
applies  his  strength.  A  man  of  ordinary  strength  lifting  weights 
with  his  hands  can  perform  about  1500  units  of  work  per  minute, 
and  continue  it  eight  hours  a  day ;  by  working  at  a  vertical  crank 
or  windlass,  he  can  perform  2500  units  of  work  per  minute ;  when 
in  good  position  for  rowing,  so  that  the  muscles  of  the  back,  arms, 
and  legs  are  all  well  employed,  he  can  perform  4000  units  of  work 
per  minute,  and  continue  it  for  several  hours.  In  trials  of  strength 


406  MECHANICS. 

which  are  to  continue  but  for  a  few  minutes,  this  amount  of  work 
is  greatly  exceeded.  Steam-engines  vary  from  less  than  TOO  to 
more  than  1000  horse-power. 

gow  ft  1399.  The  number  of  Horse  Power  of  any 

Horse  Power      engine  or  other  motor  is  calculated  by  mul- 
tiplying together  the  number  of  pounds  of 
force  exerted  by  the  distance  in  feet  per  minute,  and 
dividing  by  33,000. 

EXAMPLES   FOR   SOLUTION. 

1400.  (1)  What  must  be  the  power  of  an  engine  to  raise 
a  block  of  stone  weighing  2000  pounds  to  a  height  of 
40  feet  in  one  minute  ? 

Ans.  2000  x  40  -r-  33000  -  2T4<fu  Horse  Power. 

(2)  What  power  is  exerted  by  a  steam-engine  when  the 
steam  pressure  on  the  piston  is  4000  pounds  and  the  engine 
makes  30  double  strokes  of  5  feet  each  per  minute  ? 

Ans.  36T3o60  Horse  Power. 

(3)  What  power  is  exerted  by  a  stream  of  water  falling 
20  feet,  if  600  cubic  feet  of  water  are  afforded  per  minute  ? 
(A  cu.  ft.  of  water  weighs  62^  pounds.) 

Ans.  22^  Horse  Power. 

(4)  What  power  is  exerted  by  a  locomotive  in  drawing  a 
train  whose  entire  weight  is  100. tons,  on  a  level  track,  at 
the  rate  of  20  miles  an  hour  ?     (It  requires  a  pulling  force 
of  8  Ibs.  per  ton  to  keep  cars  in  motion  on  a  level  track ; 
20  miles  an  hour  is  1760  feet  per  minute.) 

Ans.  42T7Q  Horse  Power. 

1401.  It  is  of  the  highest  importance  that  the  student  of  Me- 
chanics should  acquire  clear  conceptions  of  the  subjects  treated  in 
the  last  few  sections.     Much  fruitless  lahor  is  expended  on  inven- 
tions which   are  entirely  worthless  solely  because  the  inventor  has 
imperfect  notions  respecting  the  relation  between  force  and  work. 
The  most    common  error  is  that  of  mistaking  mere  pressure  for 

,  or  ability  to  perform  work.     This  mistake  has  recently  led 


MECHANICS.  407 

many  to  a  false  estimate  of  the  power  of  electro-magnetic  engines  ; 
the  mere  force  of  adhesion  of  an  armature  to  the  magnet  being 
employed  as  an  element  in  the  calculation  instead  of  the  distance 
through  which  the  magnet  would  draw  a  known  weight. 

Most  of  the  seekers  after  perpetual  motion,  of  whom  there  are 
still  a  large  number,  labor  under  the  delusion  that  pressure  is  iden- 
tical with  energy,  and  is  capable  of  performing  work.  The  great 
majority  of  the  ingenious  devices  for  superseding  crank  motion  in 
the  steam-engine  have  been  produced  under  the  mistaken  impres- 
sion that  the  pressure  on  the  crank-shaft  at  certain  points  during 
the  stroke  caused  a  corresponding  loss  of  work  on  the  part  of  the 
steam,  although  this  pressure  was  of  the  same  nature  as  the  force 
exerted  on  the  interior  of  the  boiler,  and  could  lead  to  no  loss  ex- 
cept so  far  as  it  caused  friction  in  the  moving  parts. 

1402,  The  law  of  equilibrium  of  the  Mechanical  Powers  or 
Simple  Machines  (see  paragraph  323)  may,  by  recognizing  the  prin- 
ciple of  work,  be  stated  thus  :  The  work  of  the  Power  is  equal  to 
the  work  of  the  Weight ;  or  the  Power  multiplied  by  the  distance 
through  which  it  moves  is  equal  to  the  Weight  multiplied  by  the 
distance  through  which  it  moves. 

What  it  meant       1403' .  PerPetual  Motion,  so   long   sought 
by  Perpeti 
Motion  ? 


by^Perpetual     for,  signifies  in  Mechanics  a  machine  which 


will  continue  in  motion  until  some  of  its 
parts  wear  out,  without  the  aid  of  any  external  communi- 
cated force. 

This  definition  does  not  include  some  machines  which  can  be 
made  to  run  continuously  until  some  portion  wears  out — such  as 
Wind-mills  in  the  region  of  the  Trade- winds ;  Tide  Motors,  and 
Floating-water  Mills  in  our  rivers  that  never  run  dry,  for  all  of 
these  are  kept  in  motion  by  forces  external  to  the  machine. 

A  wheel  so  delicately  adjusted  as  to  continue  in  motion  for  an 
indefinite  period  from  a  single  impulse,  would  be  a  perpetual  mo- 
tion according  to  the  definition  ;  but  even  the  best  adjusted  wheel 
is  soon  brought  to  rest  by  the  common  impediments  to  motion — 
friction  and  resistance  of  the  air  ;  so  even  this  is  clearly  impossible. 

But  perpetual-motion  seekers  look  for  something  more  than  con- 
tinuous motion  ;  they  hope  to  discover  a  form  of  mechanism  which 
will  be  capable  not  only  of  continuous  motion  in  itself,  but  will  be 
able  to  afford  power  to  turn  other  machines  ;  that  is,  a  machine  is 
wanted  which  shall  create  power  or  perform  more  work  than  is 
required  to  set  it  in  motion.  As  this  is  in  opposition  to  the  law  of 
equilibrium  of  simple  machines,  such  an  invention  is  far  beyond 
the  bounds  of  possibility. 

Define  Centre        1404.   The  Centre  of  Gravity  of  a  body  is 
of  Gravity.        t^at  pomt  Wnici1?  if  supported,  the  body  will 

remain  at  rest  in  any  position. 


408 


MECHANICS. 


A  body  at  rest  is  said  to  be  in  a  condition  of  Equili- 
brium ;  which  means  simply  that  the  external  forces  act- 
ing upon  it  balance  each  other.  These  forces  in  most 
bodies  are  Gravity  and  the  resistance  of  the  point  of 
support. 

1405.  There  are  three  states  of  Equili- 
brium of  such  supported  bodies,  called  Stable, 
Unstable,  and  Indifferent  Equilibrium. 

1406.  A  body  is  in  a  condition  of  Stable 
Equilibrium  when,  if  it  is  slightly  tipped,  it 

tends  to  return  to  its  former  position. 

Fig.  203. 


How  many 
kinds  of 
Equilibrium 
are  there  ? 

Define  Stable 
Equilibrium. 


1407.  The  table  shown  in  Fig.  203  is  in  Stable  Equili- 
brium, as  the  line  of  direction  (230)  from  g  falls  between 
the  points  of  support ;  so  that  if  it  were  considerably  in- 
clined it  would  tend  to  return  to  its  position. 

1408.  The  toy  figure  represented  in  two  positions  in 
Fig.  204  represents  also  a  case  of  Stable  Equilibrium ;  a 
bullet  in  the  base  of  the  figure,  which  is  otherwise  made 
of  light  material,  brings  the  toy  upright  from  any  position 
in  which  it  may  be  placed. 


MECHANICS. 
Fig.  204, 


409 


1409.  Fig.  205  represents  another  common  toy  illustrat- 
ing the  same  thing.  The  ball  on  the  wire  attached  to  the 
body  of  the  horse  is  considerably  heavier  than  the  horse 
and  rider.  By  bending  the  wire  slightly,  the  figure  may 
be  made  to  assume  different  positions. 

Fig.  205. 


Fig.  206. 


In  the  example  shown  m  Fig.  205  it  will  be  noticed  that 
the  centre  of  gravity  is  below  the  point  of  support ;  in  the 
former  examples  it  was  above. 

1410.  A  similar  experiment  is  shown  in  Fig.  206.  A 
cork  and  wire,  or  needle,  are  rendered  stable  when  the  wire 


410 


MECHANICS. 


rests  on  its  point,  by  sticking  knives  in  the  cork  in  such 
a  manner  that  the  centre  of  gravity  of  the  whole  shall  be 
lower  than  the  point  of  support. 

Define  Unstable  1411.  A  body  is  said  to  be  in  a  condi- 
Equilihrium.  tion  of  Unstable  Equilibrium  when,  if  it  be 
slightly  disturbed,  it  will  turn  over  to  a  new  position. 

Fig.  207. 


1412.  A  stick  balanced  as  in  Fig.  207  represents  this 
condition.    The  centre  of  gravity  must  be  maintained  care- 
fully over  the  point  of  support  or  the  stick  will  fall  over. 

1413.  It  will  readily  be  inferred  that  there  may  be  dif- 
ferent conditions  or  degrees  of  stability  in  the  same  body. 
A  brick  is  the  most  stable  when  resting  on  its  broadest 
side,  and  the  least  stable  when  standing  on  its  end. 
Define  Indiffer-        1414.  A  body  is  said  to  be  in  a  condi- 
ent  Equilibrium.  tion  of  Indifferent  Equilibrium  when,  upon 
being  slightly  moved,  it  has  no  tendency  to  move  further, 


MECHANICS. 


411 


or  to  return  to  its  former  position.  A  well-balanced  wheel 
upon  an  axis,  or  a  sphere  of  uniform  density  upon  a  level 
surface,  are  examples  of  it. 

1415.  The  Balance  is  a  lever  with  equal 
arms,  having  a  scale-pan  attached  to  each, 
and  a  fulcrum  about  which  the  arms  turn  with  the 
slightest  possible  friction. 

Fig.  208. 


What  is  a 
Balance  ? 


1416.  When  there  is  no  weight  in  the  scale-pans,  the  centre  of 
gravity  of  the  pans  and  beam  should  be  a  short  distance  below  the 


412  MECHANICS. 

fulcrum.  If  the  centre  of  gravity  were  exactly  at  the  fulcrum,  the 
balance  would  be  in  a  state  of  indifferent  equilibrium,  and  the 
scale-pans  would  remain  at  rest  when  they  were  not  on  the  same 
level. 

If  the  centre  of  gravity  were*  above  the  fulcrum,  the  balance 
would  be  in  unstable  equilibrium,  and  the  beam  BA  would  turn 
upside  down. 

When  the  centre  of  gravity  is  too  far  below  the  fulcrum,  the 
balance  is  not  sufficiently  sensitive ;  so  a  heavily-loaded  balance  is 
moved  but  little  by  slight  additions  to  the  load  of  either  pan. 

When  a  balance  is  in  perfect  adjustment,  the  arms  are  of  pre- 
cisely the  same  length,  and  the  pans  unloaded  exactly  balance  each 
other.  It  is  difficult  to  keep  a  balance  in  this  condition  ;  and  al- 
though one  arm  is  made  so  that  its  length  can  be  slightly  altered 
by  a  screw,  still  the  frequent  adjustment  of  a  delicate  balance  is 
very  tedious. 

Weighing  correctly,  however,  may  be  accomplished  by  a  balance 
even  when  the  arms  are  not  of  the  same  length,  provided  that  there 
is  the  proper  degree  of  nicety  in  the  construction  of  the  fulcrum. 

Explain  double  1417.  The  method  is  called  that  of 
weighing.  "double  weighing,"  and  is  performed  as 

follows : 

Suppose  it  were  required  to  weigh  out  500  grains  of  a  certain 
substance  in  the  balance  represented  in  Fig.  208.  First  put  in  one 
scale-pan,  as  C,  for  instance,  the  standard  weights  to  the  amount  of 
500  grains ;  then  in  the  scale-pan  D  put  any  heavy  dry  material, 
such  as  shot,  until  the  two  pans  balance,  as  indicated  by  the  pointer 
a.  Remove  the  weights  from  the  scale-pan  C,  and  add  the  sub- 
stance to  be  weighed  until  the  balance  is  again  restored ;  the  cor- 
rect weight  is  thus  insured. 

It  is  evident  that  to  produce  the  same  effect  as  the  standard 
weights,  and  in  the  same  position  on  a  lever,  the  substance  must 
have  the  same  weight,  even  if  the  arms  are  of  unequal  length. 

What  are  the  1418.  HYDROSTATICS.  The  properties  of 
™np*rtiwrofnt  li(luids  that  are  of  Primary  importance  in 
liquids?  Mechanics  are  incompressibility  and  the 

power  of  transmitting  force  equally  in  all  directions. 

To  what  extent  1419'  Liquids  are  not  absolutely  incom- 
es water  com-  pressible.  Water  is  compressed  -B-JJTJ  of  its 
pressiblef  ,  *,  _,  CAA  £* 

volume  by  a  pressure  of  1500  Ibs.  to  the 

square  inch. 

1420.  The  ease  with  which  pressure  is  transmitted  in  all 


MECHANICS. 


413 


directions  is  owing  to  the  ease  with  which  the  molecules 
*move  over  each  other. 


Explain 
Fig.  209. 


1421.  If  a  bottle  be  filled  with  water  and 
a  cork  fitted,  as  in  Fig.  209,  then  any  pres- 
sure exerted  upon  the  cork  is  transmitted  to  all  interior 
portions  of  the  bottle. 

If  the  area  of  the  cork  be  one  square  inch,  and  the  pres- 
sure upon  it  be  10  pounds,  then  a  pressure  of  10  pounds  is 
exerted  upon  every  square  inch  of  surface  on  the  interior 
of  the  bottle,  and  also  upon  every  square  inch  throughout 
the  liquid. 

Fig.  209. 


1422.  This  property  may  be  conveniently  employed  in  changing 
the  direction  of  a  pressure  and  transmitting  it  to  a  distance. 

If  a  force  be  applied  at  0  (Fig.  210)  to  a  piston  fitted  to  a  bent 
tube  filled  with  water,  the  same  force  will  be  exerted  upon  the 
piston  P,  provided  the  tube  itself  be  firmly  fixed.  The  pressure 
would  be  transmitted  equally  well  if  the  tube  were  bent  entirely 
around  in  a  circular  or  spiral  form. 

Use  has  been  mado  of  this  principle  in  the  construction  of  a 


414 


MECHANICS. 


short  line  of  telegraph — an  iron  tube  filled  with  water  serving  as  a 
medium  for  conduction  of  the  signals. 

1423.  It  must  be  borne  in  mind  that  it  is  simple  pressure  that  is 
transmitted  without  loss  ;  if  the  force  were  applied  so  as  to  perform 
work — that  is,  if  the  whole  body  of  water  were  urged  through  the 
tube — then  there  would  be  a  sensible  loss  of  energy  or  power  at 
the  further  end  of  the  tube,  in  consequence  of  friction  of  the  sides 
of  the  tube,  and  also  by  reason  of  change  of  direction  at  the  bends. 


Explain 
Fig.  211. 


1424.  One  of  the  results  of  the  equality  of 
pressure  of  liquids  is,  that  vessels  communi- 
cating with  each  other,  as  in  Fig.  211,  the  liquid  rises  to 
the  same  level  in  all  the  vessels,  whatever  their  relative 
size. 

Fig.  211. 


As  the  pressure  in  any  column  of  particles  in  either  of 
these  vessels  arises  from  the  weight  of  the  column  only,  it 
follows  that  the  pressure  in  all  the  columns  at  the  same 
depth  is  precisely  the  same,  so  that  the  pressures  trans- 
mitted in  all  directions  from  any  particle  below  the  surface 
are  exactly  balanced  by  other  pressures  from  neighboring 
particles.  (See  paragraph  443.) 


MECHANICS. 


415 


1425.  The  Bramah  Press,  represented  in  section  in 
Fig.  68  and  described  on  page  121,  is  shown  in  Fig.  212 
with  a  complete  equipment  of  working  parts. 


Fig.  219. 


A  represents  the  pump  cylinder,  and  R  the  press  cylin- 
der. The  pump,  which  is  worked  by  aid  of  the  lever- 
handle  0  and  piston-rod  a,  is  in  communication  with  some 
supply  of  water  not  shown  in  the  figure.  The  pump  fills 
with  water  at  each  up-stroke  of  the  handle ;  this  is  then 
forced  at  the  down- stroke  through  the  pipe  d  into  the 
large  cylinder.  The  ordinary  valves  of  a  forcing-pump  are 
employed  to  prevent  communication  between  the  pump 
and  press-cylinder  except  during  the  down-stroke. 


416  MECHANICS. 

«.  1426.  It  frequently  happens  that  the  enormous  pressures  applied 
in  the  hydrostatic,  bursts  one  of  the  cylinders,  most  frequently  the 
press  cylinder.  At  such  times,  although  the  pressure  is  many  times 
greater  than  that  in  ordinary  steam-boilers,  whose  explosions  are  so 
disastrous,  the  bursting  of  the  press  cylinder  is  attended  with  no 
danger  to  people  standing  near  it.  The  reason  of  the  difference  lies 
in  the  different  physical  properties  of  the  steam  and  water.  Steam 
is  highly  elastic  and  compressible,  and  when  set  free  from  pressure 
expands  largely,  exerting  all  the  time  its  elastic  force  ;  hence  it 
throws  fragments  of  the  boiler  at  the  time  of  explosions  to  great 
distances.  Water,  on  the  other  hand,  is  compressed  only  to  a  small 
extent  by  the  greatest  pressures  to  which  it  is  ever  subjected,  so 
that  the  slightest  escape  of  the  water  is  sufficient  to  relieve  it  of  all 
its  energy. 

The  largest  hydrostatic  presses  in  this  country  have  a  pump- 
cylinder  of  one  inch  diameter  and  a  press-cylinder  of  twenty  inches 
interior,  and  about  forty  inches  exterior  diameter.  By  the  aid  of  a 
small  engine  to  work  the  pump,  such  machines  can  exert  a  pres- 
sure of  2000  tons. 

1427.  Specific  Gravity.  —  When  a  body  is  twice  as  heavy 
as  the  same  bulk  of  water,  we  say  it  has  a  Specific  Gravity 
of  2;  if  it  be  five  times  as  heavy  as  water,  its  Specific 
Gravity  is  said  to  be  5. 

Define  Specific  Specific  Gravity,  therefore,  is  the  weight 
Gravity.  Of  a  k0(jy  compared  with  the  weight  of  the 

same  bulk  of  water.     This  applies  to  solids  and  liquids. 


ffowisthespe-       1428'    The  sPecific  8*^  of  an^  lictuid 
cific  gravity  of  is  easily  found  by  weighing  a  bottle  full  of 

the  liquid,  and  then  the  same  bottle  filled 
with  water. 

The  bottle  must  be  accurately  filled  to  the  same  height 
in  both  experiments,  and  allowance  must  be  made,  of 
course,  in  both  cases  for  the  weight  of  the  bottle. 

Having  completed  the  weighing,  divide  the  weight  of 
the  liquid  by  the  weight  of  the  water. 

1429.  Specific-gravity  bottles  are  made  for  this  purpose,  which 
hold,  when  filled  to  a  mark  on  the  neck,  exactly  1000  grains  of  dis- 
tilled water,  so  that  the  experimenter  who  is  furnished  with  one 
can  save  one  half  of  the  labor  of  the  above  experiment. 


MECHANICS. 


417 


The   specific   gravity  of  liquids   is 

construction  of  more    rapidly,   but    rather    less    accurately 
the  Hydrometer.  „          ,  ,         .  n     „  . ,       TT    -. 

formed  by  aid  of  the  Hydro-         Fig  213 

meter  (Fig.  213).  It  consists  of  a  closed 
glass  tube,  having  a  double  bulb  at  one 
end;  the  lower  or  end  bulb  contains  shot 
or  mercury,  to  cause  the  tube  to  float  up- 
right. The  tube  above  the  bulbs  contains 
a  strip  of  paper  with  marks  indicating  the 
specific  gravity.  It  sinks  deepest  in  light 
liquids,  so  the  smaller  numbers  will  be 
found  at  the  top  of  the  tube.  The  marks 
are  first  adjusted  by  trial  of  the  instrument 
in  liquids  of  known  specific  gravity.  When 
employed  for  alcohol  only,  this  instrument 
is  called  the  Alcoholmeter,  and  is  marked  to  indicate  the 
percentage  of  alcohol  in  the  dilute  mixtures.  When  em- 
ployed in  determining  the  specific  gravity  of  milk,  it  is 
sometimes  called  the  Lactometer. 

1431.  In  all  accurate  experiments,  the  temperature  must  be  care- 
fully noted,  and  if  readily  practicable  brought  to  62°  Fahrenheit. 

Explain  1432.  The  principle  employed  in  determining 
Fig.  214.  tfte  specific  gravity  of  solids  may  be  best  compre- 
hended by  aid  of  the  following 
illustration.  (See  Fig.  214.) 

The  block  abed  is  immersed 
in  water.  The  pressure  exerted 
upon  it  by  the  particles  of  water 
about  it  is  just  such  as  would  be 
sufficient  to  hold  at  rest  an  equal 
bulk  of  water,  for  if  there  were 
water  in  the  place  of  the  block,  it 
would  be  held  in  equilibrium. 


Fig.  214. 


418  MECHANICS. 

If,  therefore,  the  block  weighs  just  as  much  as  an  equal 
bulk  of  water,  it  will  remain  stationary  ;  if  it  is  heavier,  its 
weight  will  overbalance  the  pressure  of  the  water  particles, 
and  it  will  sink  ;  if  it  is  lighter,  it  will  rise  ;  but  in  any 
case  it  will  be  pressed  upward  with  a  force  equal  to  the 
weight  of  the  same  bulk  of  water. 

1433.  The  loss  of  weight  which  a  heavy 
What  loss  of  &  / 

weight  is  sus-      body  sustains,  then,  when  held  suspended  in 

the  water>  is  ihQ  weiShfc  of  an  e(lual  bulk  of 
water. 


Bepeat  the  Rule  1434  To  find  its  sPecific  faYU7>  then> 
for  specific  weigh  it  in  the  manner  described  on  page 

126,  and  divide  its  true  weight  by  its  loss  in 

water. 

What  is  1435.  A  body  lighter  than  water  is  pressed 

Buoyancy?       upward  when  immersed  by  force  equal  to 
the  difference  between  the  weight  of  the  body  and  the 
weight  of  an  equal  bulk  of  water  ;  this  force  is  called  the 
buoyancy  of  the  body. 
What  is  the          1436.  The  specific  gravity  of  such  a  body 

^{%ofK$t  is  found  by  dividing  the  weight  °f  the  body 

solids  ?  .  by  its  weight  added  to  its  buoyancy. 

The  buoyancy  is    found  by  determining    how  much 
weight  is  required  to  sink  the  body.     (See  page  127.) 

Define  1437.  Hydraulics  treats  of  the  laws  gov- 

Hydraulws.  erning  liquids  in  motion,  and  of  the  useful 
application  of  these  laws  in  the  employment  of  water  as  a 
motive  power,  the  water  supply  and  drainage  of  cities,  and 
the  improvement  of  rivers  and  harbors. 

How  is  water  em-  !438.  Water  is  employed  as  a  motive 
ployed  for  power*  power  ky  utilizing  the  energy  of  its  fall 
as  it  descends  the  rivers  on  its  way  to  the  sea. 


MECHANICS. 


419 


1439.  The  water  power  of  running  streams  is  made  useful  by 
selecting  a  place  on  the  stream  where  the  fall  is  sufficiently  rapid, 
and,  if  necessary,  building  a  dam  to  secure  a  vertical  fall.  From 
the  store  of  water  thus  held  back  in  the  stream,  an  artificial  channel 
or  flume  conducts  the  water  to  the  water-wheel. 


Mention  the  ' 
different  forms 
of  Water-wheels. 


When  is  an 
Overshot  Wheel 
used  f 


1440.  Water-wheels  are  of  four  different 
kinds:    Overshot,   Undershot,   Breast    and 
Turbine.     (See  page  82.) 

1441.  The  Overshot  Wheel  is  employed 
when  the  stream  is  small  and  the  fall  high. 
The  flume  is  continued  Fig.  215. 

out  over  the  wheel,  as  represented  in 
the  figure,  and  the  buckets  are  filled 
in  succession. 

The  wheel  being  overloaded  on  one 
side,  turns  with  a  force  proportioned 
to  this  extra  weight.  The  whole  force 
of  the  falling  water  can  never  be  util- 
ized by  this  means,  even  if  all  the  water  of  the  stream  falls 
into  the  buckets,  for,  as  may  be  seen  from  the  figure,  a  por- 
tion of  the  water  falls  from  the  bucket  again  before  it  has 
reached  its  lowest  point. 

Fig.  216. 

1442.  The  Breast 
Wheel  is  adapted  to 
a  larger  supply  of 
water  and  a  lower 
fall.  The  water  is  re- 
ceived at  about  half 
the  height  of  the 
wheel  in  buckets  or 
upon  floats.  If  the 
water  comes  upon  the 
buckets  above  the 
centre,  it  is  called  a  high-breast  wheel ;  if  below,  a  loiv- 
18 


420 


MECHANICS. 


breast  wheel.  The  loss  of  power  occurs  here  as  in  the 
Overshot  Wheel  —  neither  of  them  affording  more  than 
75  per  cent,  of  the  real  power  of  the  stream. 

The  Undershot  Wheel  is  a  ruder 


What  is  said  of 

the  Undershot      kind  of  motor  ;   it  is  furnished  with  floats, 

Wheel?  n   .          .     ..  „  , 

or    flat   projecting    surfaces    only,    against 


Fig.  217. 


which  the  running  water 
impinges,  and,  by  aid  of 
the  momentum  acquired  in 
its  previous  descent,  affords 
power  to  turn  the  wheel. 
Only  20  per  cent,  of  the 
power  of  the  water  is  ren- 
dered effective  by  this  kind 
of  wheel. 

What  is  said  of      1444.  The  Turbine  Wheel  is  by  far  the 
the  Turbine?       best  of  all  the  hydraulic    motors.      It   13 

readily  made  to  utilize 
from  80  to  90  per  cent,  of 
the  power  of  the  stream. 

1445.  It  is  placed  hori- 
zontally, and  is  entirely 
immersed  in  the  water. 


Fig.  218. 


Fig.  218  represents  a  plan 
of  the  wheel.  Fig.  219 
is  an  elevation  or  verti- 
cal section  of  the  same 
wheel. 

The  flume  conducts 
Describe  Figs,  the  water  vertically  upon  the  wheel ;  the  size 
218  and  219.  Of  |he  flume  \s  equal  to  the  size  of  the  cen- 
tral portion  of  the  wheel  marked  a  a  a  in  Fig.  218  and 


MECHANICS. 


421 


G  G  in  Fig.  219.  The  buckets  are  shown  at  vvv.  The 
spiral  lines  marked  a  a  a  represent  curved  partitions  or 
guide-curves  fixed  in  the  flume,  and  form  no  part  of  the 


wheel ;  they  serve  to  give  such  direction  to  the  descending 
current  as  to  enable  it  to  act  to  the  best  advantage  on  the 
buckets. 

1446.  It  will  be  seen  by  examining  Fig.  219  that  the  water  de- 
scending the  flume  D  G  is  prevented  from  passing  directly  down- 
ward by  the  lower  plate  HUH,  and  has  no  other  outlet  than  the 
space  between  P  and  H,  where  the  buckets  are  placed.     The  shaft 
E  E  turns  with  the  wheel,  and  affords  the  means  of  communication 
with  such  machinery  as  the  wheel  is  designed  to  carry. 

The  above  figures  represent  the  Fourneyron  Turbine,  so  called 
from  the  inventor. 

1447.  There  is  another  variety  known  as  Centrevent  Turbines, 
from  the  fact  that  the  water  flows  in  at  the  circumference,  and 
escapes  at  the  centre.     Fig.  220  represents  one  of  this  class.     The 
water  is  conducted  from  the  vertical  flume  C  by  a  horizontal  con- 
duit at  the  bottom,  entirely  around  the  wheel.     The  only  escape 


422 


MECHANICS. 


for  the  water  is  between  the  buckets  and  cut  at  the  central  open- 
ings marked  /  at  the  top  and  bottom  of  the  wheel. 

Fig.  220. 


1448.  Turbine  Wheels  are  always  made  of  iron,  and  are  so  accu- 
rately made  that  no  sensible  leakage  occurs  between  the  fixed  and 
movable  portions  (as  at  G,  Fig.  219). 

1449.  Turbines  are  much  smaller  than  other  water-wheels  de- 
signed for  the  same  streams.     It  is  not  uncommon  to  replace  an 
overshot  wheel  twenty  feet  in  diameter  and  six  feet  wide  by  a  tur- 
bine only  three  feet  in  diameter  and  six  inches  deep,  and  obtain 
greater  efficiency  by  the  change. 

What  is  the  1450.   It  should  be  carefully  remembered 

Uyidded^byea       that  no  Und  of  hydraulic  motor  can  afford 
stream?  any  more  power  than  is  due  to  the  weight  of 

the  water  yielded  ly  the  stream  descending  through  the 
height  of  the  fall. 

How  is  the  power  of  a      1451.  The  theoretical  horse-power  of 
stream  calculated?       a  stream  is  found  by  a  survey  which 


MECHANICS. 


Fig.  221. 


measures  the  amount  of  water  flowing  by  a  particular 
point  in  one  minute,  and  the  amount  of  fall  that  can  be 
made  available. 

The  number  of  pounds  per  minute  multiplied  by  the  number  of 
feet  fall,  and  the  product  divided  by  33,000,  determines  the  entire 
horse-power  developed  by  the  stream ;  the  percentage  of  this 
amount  utilized  depends  upon  the  kind  of  wheel  used. 

Describe  1452.    Barker's  Mill,  represented  in  Fig. 

Barker's  Mitt.  ^21,  is  frequently  employed  as  a  motor.  If 

water  be  supplied  so  fast 
as  to  keep  the  central 
tube  DC  filled,  there  will 
result  a  hydrostatic  pres- 
sure within  the  curved 
arms  at  the  bottom. 
This  pressure  is  in  all 
directions ;  the  arms  be- 
ing opened  so  as  to  dis- 
charge the  water  in  one 
direction  from  each  arm, 
the  pressure  is  relieved 
on  that  side,  but  remain- 
ing on  the  opposite,  it 
gives  motion  to  the  arms. 

1453.  It  is  popularly  be- 
lieved that  Barker's  Mill 
runs  by  aid  of  a  push  of  the 
outflowing  jet  against  the 
air ;  but  the  fact  that  it  runs 
better  under  an  exhausted 
receiver  than  in  the  open 
air,  serves  to  dispel  this 
impression!  Small  Barker's 
Mills  are  made  in  order  to 
show  this  experiment.  (See 
Fig.  98) 

are  cities  sup-      1454.  The  water  supply  of  cities  is  ac- 
uiih  water?    complished  by  providing   a   reservoir  of 


424 


MECHANICS. 


water  higher  than  the  dwellings,  and  from  this  reservoir 
distributing  the  water  through  the  streets  in  iron  pipes. 

In  some  cities,  as  Boston  and  New  York,  the  water  comes 
from  natural  sources  several  miles  off,  but  high  enough  to 
answer  the  purpose  of  distribution.  In  Brooklyn,  Philadel- 
phia, and  Chicago  the  water  is  pumped  up  into  reservoirs 
which  have  the  required  height. 

1455.  The  distribution-pipes  are  laid  about  the  streets  at  a  suffi- 
cient depth  to  provide  against  freezing.  The  sizes  for  different  dis- 
tricts are  fixed  in  accordance  with  the  known  principles  of  Hydrau- 
lics. The  water  can,  of  course,  rise  no  higher  in  pipes  in  the  houses 
than  the  surface  of  the  water  in  the  reservoir.  It  very  rarely  rises 
as  high,  in  consequence  of  the  constant  flow  of  water  out  of  the 
pipes  at  lower  levels. 

The  friction  of  the  sides  of  the  pipe  and  the  loss  of  force  by 
change  of  direction  at  the  bends,  are  serious  impediments  to  the 
flow  of  water  through  pipes. 

Fig.  222. 


1456.  Fig.  222  represents  a  reservoir  and  a  foun- 
Fig.  222.  tain  supplied  by  it.  If  a  pipe  were  extended  high 
enough,  from  the  centre  of  the  pond,  in  place  of  the  foun- 


MECHANICS. 


425 


tain,  the  water  would  slowly  rise  to  the  height  of  the  water 
in  the  reservoir.  The  fountain  jet  will  not  rise  so  high, 
both  on  account  of  the  friction  of  the  pipe  and  resistance 
of  the  air.  The  farther  the  fountain  is  from  the  reservoir, 
the  less  will  the  height  of  the  jet  be,  because  the  friction  is 
proportioned  to  the  length  of  the  pipe. 

1457.  When  a  liquid  is  allowed  to  flow  through  an  ori- 
fice in  the  side  or  bottom  of  a  vessel,  it  is  found  by  experi- 
ment that  the  shape  of  the  outlet  influences  very  largely 
the  rapidity  of  the  discharge. 

1458.  Fig.  223  represents  different  forms  of  orifices,  and 
aids  to  explain  the  difference  in  their  action. 

Fig.  223. 


tl! 


I  '// 


Explain  In  the  first,  it  will  be  seen  that  the  currents  (rc- 
Fig.  223.  presented  by  broken  lines),  coming  from  the  oppo- 
site sides  of  the  outlet,  oppose  each  other,  so  that  the  velo- 
city is  somewhat  checked ;  the  result  is  a  contraction  of  the 
outflowing  stream  to  a  size  considerably  less  than  the  ori- 
fice. The  contraction  is  found  to  occur  at  a  distance  from 
the  orifice  varying  from  half  its  diameter  to  the  whole 
diameter. 

In  the  second  form  of  orifice  the  currents  do  not  oppose 
each  other,  and  a  better  flow  of  liquid  is  the  result ;  the 
stream  is  the  whole  size  of  the  orifice,  and  flows  with  great 
emoothness. 


426  MECHANICS. 

The  third  is  the  most  unfavorable  form  for  a  free  dis- 
charge, as  currents  are  formed  in  the  vessel  having  a  direc- 
tion nearly  opposite  to  the  discharge. 

The  discharge  is  also  influenced  by  nearness  to  the  side 
of  the  vessel.  In  No.  4  it  will  be  seen  that  the  current 
would  be  deflected  by  the  momentum  of  particles  from  one 
side. 

ox,-  „•„  /*„  „«.*.•£..  1459.  To  calculate  the  velocity  and  hence 
How  is  the  quantity  th  tit  of  n  id  discharged  from  any  ori- 

falcula^  9  fice'  we  employ  the  principle  (explained  on 
page  129,  note)  that  the  velocity  of  the  stream 
is  equal  to  that  acquired  by  a  body  falling  through  a  height  equal 
to  the  depth  of  the  liquid.  The  velocity  is  therefore  calculated  by 
multiplying  the  square  root  of  the  depth  in  feet  by  8.  The  result 
expresses  the  velocity  in  feet  per  second ;  but  by  reason  of  the 
various  resistances,  the  velocity  is  generally  only  about  76  per  cent, 
of  this  calculated  amount. 

PNEUMATICS. 

1460.  Recent  applications  of  the  principles  of  Pneumatics 
invest  this  branch  of  Philosophy  with  a  new  interest. 

TP-T    ,  , .       «       The  compressibility  and  perfect  elasticity  of 

^reoTsmd'l      the  air  all°^  of  its  e™nom*cal  use  in  the  pro- 

•     i/  *      •"  9    pulsion  of  engines,  wherever  the  means  of  con- 

use  in  Mechanics?    gensation  areg  readilv  obtained.     The  drilling 

machinery  of  the  Mont  Cenis  and  Hoosac  Tunnels  were  operated  by 
the  expansive  property  of  compressed  air.  The  condensation  was 
effected  by  power  obtained  from  water-wheels  outside  of  the  tunnel, 
and  conducted  through  tubes  to  the  machines  to  be  operated.  The 
work  could  not  have  been  as  efficiently  done  by  any  other  known 
source  of  power. 

The  air,  after  doing  its  work,  served  to  ventilate  the  tunnel ;  if 
steam  power  had  been  employed,  a  large  excess  of  the  power  neces- 
sary to  perform  the  work  would  have  been  required  to  afford  the 
proper  ventilation  to  the  workmen. 

1461.  Recent  experiments  have  demonstrated  the  practicability 
of  conveying  the  compressed  air  fifteen  or  twenty  miles  through 
pipes  from  the  locality  where  the  air  is  compressed  to  the  engines 
which  it  is  to  be  employed  to  drive.     The  project  of  employing  a 
portion  of  the  enormous  store  of  waste  power  of  Niagara  to  com- 
press air  to  be  used  at  Buffalo,  twenty  miles  distant,  has  been  seri- 
ously entertained. 

1462.  Diving-bells  of  larger  dimensions  than  ever  before  em- 
ployed have  played  an  important  part  in  the  work  of  preparing  the 
foundations   for  the  East  River  Bridge,  between  New  York  and 


MECHANICS.  427 

Brooklyn.  Instead  of  the  ordinary  diving-bell  described  on  page 
151,  an  enormous  box,  or  ccvisson,  was  made,  164  feet  long  and  102 
feet  wide,  and  launched  into  the  water  open  side  downward.  After 
being  floated  to  the  proposed  site  of  the  bridge  pier,  it  was  gradu- 
ally sunk  to  the  bottom  by  the  masonry  laid  on  top.  Ir.on  tubes  or 
shafts  had  been  previously  prepared  projecting  through  the  top  of 
the  caisson,  which  was  several  feet  thick ;  to  provide  for  the  en- 
trance of  the  workmen  and  discharge  of  the  material,  air  was  forced 
in  by  air-pumps  (worked  by  steam  on  shore)  to  such  a  pressure  that 
the  water  was  forced  out,  and  the  workmen  stood  on  the  bottom. 
Seventy  or  eighty  men  at  a  time  worked  with  ease  in  this  caisson. 
The  material  dug  from  the  bottom  allowed  the  caisson  to  sink  by 
degrees  until  a  depth  of  forty-live  feet  from  the  surface  of  high 
water  was  reached.  The  masonry  upon  the  top  was  built  fast 
enough  to  keep  the  surface  above  water.  When  its  final  resting- 
place  was  reached,  the  interior  was  filled  with  concrete,  and  the 
caisson  left  to  form  the  base  of  the  granite  pier. 

Many  interesting  phenomenon  were  noticed  belonging  to  the 
denser  atmosphere  in  the  chamber.  Sounds  were  heard  more  dis- 
tinctly ;  a  painful  pressure  on  the  drum  of  the  ear  gradually  passed 
away ;  breathing  seemed  at  first  slightly  difficult ;  candle  flames 
burned  with  a  great  deal  of  smoke  ;  a  workman  who  had  occasion 
to  go  under  water  in  a  pit  in  the  bottom  of  the  caisson  found  he 
could  remain  for  an  unusual  length  of  time  without  the  least  in- 
convenience. 

At  St.  Louis,  where  an  iron  caisson  was  sunk  to  the  depth  of 
110  feet,  it  was  found  necessary  to  change  workmen  at  the  greatest 
depth  at  intervals  of  two  hours,  as  longer  stay  induced  temporary 
paralysis.  At  this  caisson  the  compressed  air  was  applied  to  an- 
other purpose.  Tubes  reaching  from  the  sand  and  water  at  the 
bottom  were  extended  upwards  to  the  outer  air ;  another  set  of 
tubes  conducted  the  compressed  air  just  into  the  lower  open  ends 
of  the  first-mentioned  tubes  ;  the  strong  upward  current  of  air  drew 
with  it  sand  and  water  at  a  rate  that  precluded  the  necessity  of  any 
other  means  of  dredging,  and  all  of  the  material  for  fifty  feet  of 
doscent  was  removed  by  this  means. 

This  dragging  action  of  a  current  of  air,  when  urged  over  liquids, 
or  even  over  bodies  of  air,  has  been  lately  utilized  in  several  ways. 

Currents  in  the  ocean  under  the  influence  of  strong  winds  are  a 
phenomena  of  the  same  kind.  It  seems  to  be  an  exhibition  of  the 
property  of  adhesion. 

1463.  Heat  is  a  motion  of  the  minute  par- 
ticles or  molecules  of  a  body.    All  kinds  of 
bodies,  whether  animal,  vegetable,  or  mineral,  and  in  any 
condition  of  matter — solid,  liquid,  or  gaseous — possess  this 
motion  in  their  molecules.    When  this  motion  is  less  than 
usual  in  any  particular  body,  we  say  the  body  is  cold ;  but 
we  know  of  no  instance  of  the  entire  absence  of  heat.    Hot 
lb* 


428  MECHANICS. 

bodies  are  those  whose  molecules  are  vibrating  with  great 
rapidity. 

Define  1464.  When  a  hot  body  is  placed  in  con- 

Conduction.  ^ac^  with  a  colder  one,  the  molecular  motion 
of  the  former  is  gradually  imparted  to  the  latter,  begin- 
ning at  the  point  of  contact,  and  gradually  extending 
throughout  the  mass.  This  communication  of  heat  from 
particle  to  particle  is  termed  Conduction. 

Explain  14G5.  The  rapidity  of  conduction  is  very  differ- 
Fig.  224.  en^  jn  different  solid  bodies.  This  fact  is  ex- 
hibited by  the  apparatus  Pigi  224> 
represented  in  Fig.  '#24. 
Rods  of  different  sub- 
stances are  fitted  in  the 
side  of  a  water-tight  ves- 
sel ;  to  the  end  of  each 
rod  is  attached  a  marble, 
held  by  a  bit  of  common 
wax.  The  box  being  filled  with  hot  water,  the  different 
conducting  powers  of  the  several  rods  is  approximately 
shown  by  the  shortness  of  the  time  required  in  each  case 
to  release  the  marble. 

146C.  Liquids  and  Gases  are  poor  conductors  of  heat. 
The  molecular  motion  is  not  easily  communicated  from 
one  molecule  to  another  on  account  of  the  ease  with 
which  each  moves  over  or  away  from  the  other. 

How  are  liquids  1467.    Heating    is    generally   brought 

and  gases  heated?    about  jn  t]iege  two   classes  Of  bodies  by 

contact  with  heated  solid  bodies. 

Explain  1468.  A  vessel  of  water  is  made  hot  throughout, 
Fig.  225.  jf  ]ieat  ^g  applied  at  the  bottom  of  the  vessel,  as 
represented  in  Fig.  225.  The  heat  at  the  bottom  of  the 


MECHANICS. 


429 


Fig.  225. 


vessel  extending  to  the  nearest  molecules  of  liquid,  they, 
by  reason  of  the  vibrations  imparted  to  them,  are  pushed 
a  little  further  apart,  so  that 
this  portion  of  the  liquid  is 
made  lighter  than  the  sur- 
rounding portions,  and  rises 
to  the  top ;  the  particles  tak- 
ing the  place  thus  vacated 
are  in  turn  heated,  and  con- 
tinuous currents  are  thereby 
established. 

These   currents  are  made 
visible  in  a  class-room  experi- 
ment by  adding  a  little  fine 
sulphur  to  the  water. 
What  is  the  pro-     This  process  of  heating  by  circulation  is 
cess  called  ?         called  convection. 

How  is  heat  1469.   Heat  is  transmitted  to  great  dis- 

transmitted  ^  tances  and  in  all  directions  from  hot  bodies 
by  the  vibrations  of  the  ether  which  is  sup- 
posed to  fill  all  space  ;  this  kind  of  transmission  of  heat  is 
called  radiation,  and  the  vibrations  pass  through  space 
with  the  velocity  of  186,000  miles  per  second. 

Define  Latent        1470.  When  a  body  receives  heat  from  any 

Heat,  also          source  whatever,  two  effects  are  immediately 
Sensible  Heat.  ,        ,     ,      . ,       ,    .  ,    . 

produced;  besides  being  raised  in  tempera- 
ture, it  is  expanded  in  volume.  That  portion  of  the  heat 
which  is  concerned  in  expanding  the  body  is  said  to  be 
latent,  as  it  cannot  be  detected  or  measured  at  once  by  the 
usual  methods.  That  portion  of  the  heat  which  raises  the 
temperature  is  said  to  be  sensible. 

1471.  The  amount  of  heat  necessary  to  raise  the  same 


430  MECHANICS. 

amount  of  different  substances  to  the  same  temperature 
varies  considerably.  It  requires  thirty  times  as  much  heat 
to  raise  a  certain  weight  of  water  one  degree  in  tempera- 
ture as  is  required  to  raise  the  same  weight  of  mercury 
through  the  same  temperature. 

What  is  a  1472.  A  heat  unit  is  the  amount  necessary 

Heat  Unit  f  £0  raise  One  pound  of  water  one  degree 
Fahrenheit. 

Define  *1473.    The   heat  necessary   to    raise    one 

Specific  Heat.  p0und  Of  any  substance  one  degree  in  tem- 
perature is  (when  measured  in  heat  units)  called  its  spe- 
cific heat. 

The  specific  heat  of  water  is  therefore  1 ;  of  mercury, 
^'o  ;  of  iron,  \ ;  of  copper,  about  Txr ;  and  of  nearly  all 
known  substances,  whether  solid,  liquid,  or  gaseous,  the 
specific  heat  is  less  than  that  of  water. 

1474.  In  all  practical  applications  of  heat  the  laws  of  latent  heat 
are  of  the  highest  importance.  Illustration  of  some  of  the  leading 
facts  is  afforded  in  the  following  account  of  the  behavior  of  water 
when  heated  from  below  the  freezing  to  above  the  boiling  point. 

Place  a  pound  of  ice  in  a  suitable  vessel  for  the  application  of 
heat.  Upon  heating  it  will  be  found  that  the  melting  begins  ex- 
actly at  32'  Fahrenheit,  and  that  the  water  will  remain  at  that  tem- 
perature until  all  the  ice  has  disappeared.  This  shows  that  heat 
must  be  expended  to  convert  a  pound  of  ice  at  32°  to  a  pound  of 
water  at  the  same  temperature.  By  careful  experiment  it  has  been 
found  that  this  expenditure  of  heat  is  enough  to  raise  143  pounds 
of  water  1°.  The  latent  heat  of  water  is  therefore  said  to  be  143° 
Fahrenheit. 

If  now  the  pound  of  water  be  heated  to  the  boiling  point  (212° 
Fahrenheit),  the  escaping  steam  will  be  found  to  be  of  the  same 
temperature  ;  and  although  a  long-continued  application  of  the  heat 
be  necessary  to  boil  the  water  away,  no  rise  of  temperature  above 
312J  will  be  found  in  either  water  or  steam.  Heat  has  been  neces- 
sary again  to  effect  the  change  of  state,  and  in  this  change  967  heat 
units  have  been  employed.  In  other  words,  it  requires  180  units  of 
heat  to  raise  1  lb.  of  water  from  32°  fo  212°.  and  it  requires  5-V  times 
as  much  heat  to  simply  convert  the  same  water  into  steam,  without 
raising  its  temperature  above  212". 

It  should  be  remembered  that  the  latent  heat  becomes  sensible 
when  steam  is  condensed  to  water,  or  water  converted  to  ice. 


MECHANICS.  431 

prn,    .  .    ,,  1475.  Heat  is  employed  in  various  ways  to  pro- 

\r  a  !»•  /  duce  motive  power — chiefly,  however,  in  the  steam- 
Mecnanicat  engilie>  The  relation  between  heat  and  work  has 
J^qmrnient  been  carefullv  determined,  and  is  found  to  be  as 
follows  :  "  The  heat  required  to  raise  one  pound  of 
water  one  degree  is  equivalent  to  a  force  necessary  to  raise  772 
pounds  one  foot  high  ;"  or,  more  briefly,  "  one  heat  unit  is  equiva- 
lent to  772  units  of  work." 

By  reason  of  various  losses  in  our  heat  motors,  we  do  not  re- 
alize more  than  -^  of  this  amount  even  in  our  best  steam-engines. 

OPTICS. 

What  is  1476.  Light  is  the  result  of  vibrations  in  the 
Light?  ether  which  fills  all  space.  These  vibrations  are 
of  different  degrees  of  rapidity ;  the  slowest,  which  affect 
our  senses,  we  recognize  as  heat,  as  already  explained  in  a 
previous  section ;  those  which  are  capable  of  producing 
vision  vary  also  in  their  rate  of  vibration ;  the  slowest  that 
can  affect  the  eye  being  those  that  produce  a  dull  red,  and 
the  most  rapid  a  violet  color.  These  waves  all  move  for- 
ward at  a  rate  of  186,000  miles  in  a  second.  Even  the 
longest  of  these  waves  (the  red)  are  so  minute  that  39,000 
are  included  in  a  single  inch,  while  of  the  violet  57,500  are 
contained  in  the  same  length. 

When  the  waves  are  high,  the  light  is  said  to  be  intense. 

1477.  A  Eay  is  only  the  direction  along 
Define  a  Ray.          .  .         .. 

which  the  wave  is  moving. 

Describe  the  dif-  1478.  Light  and  heat  waves  are  essen- 
{ouTd^andl^ht  tiallF  different  in  their  character  from  sound 
waxes.  waves.  The  undulations  which  produce 

sound  are  only  backward  and  forward  motions  in  the  air 
or  other  medium  which  transmit  them.  The  light  and 
heat  waves  are  vibrations  on  all  sides  of  the  line  along 
which  they  are  propagated. 

1479.  The  number  of  these  vibrations  that  enter  the  eye 
in  a  second  is  found  by  multiplying  the  number  of  inches 


432 


MECHANICS. 


in  186,000  miles  by  the  number  of  vibrations  in  a  single 
inch ;  these  for  extreme  violet  are  660  millions  of  millions.* 

How  many  1480.  Lenses  are  transparent  bodies  with 

forms  of  lenses  curved  surfaces.     They  are  generally  made 
are  there?  „     ,  n  ,      .~    ,  ,,  -. 

of  glass,  and  are  classified  generally  under 

gix  varieties. 

Fig.  226. 

M  IT 


The  double  convex  (M),  the  plano-convex  (N),  the  men- 
iscus (0),  the  double  concave  (P),  the  plano-concave  (Q), 
and  the  concavo-convex  (E). 

1481.  Lenses  thickest  in  the  middle  magnify;  those 
thickest  at  the  edges  diminish.  This  property  leads  to 
their  being  classified  under  two  classes ;  the  first  three  in 
the  above  list  being  convex,  or  magnifying  glasses,  and  the 
last  three  concave,  or  diminishing  glasses. 

Explain.       1482.  The  action  of  a  double  convex  lens  upon 
Fig,  227.   parallel  rays  of  light  is  represented  in  Fig.  227. 

Fig.  227. 


All  the  rays  that  strike  the  glass  obliquely  are  bent  from 

*  For  the  laws  of  Reflexion  of  Light,  see  816-838. 

The  principles  of  Kefraction,  as  defined  in  842,  are  best  understood  hy  con- 
sidering the  properties  of  lenses. 


MECHANICS.  433 

their  course  both  on  entering  and  leaving  the  lens.  The 
ray  X  keeps  its  direction,  as  it  is  perpendicular  to  both 
surfaces.  All  the  rays  meet  at  a  common  point,  F,  called 
the  focus. 

1483.  Practically,  this  result  is  rarely  realized.  When  the  sur- 
faces are  spherical,  the  rays  that  fall  upon  the  lens  nearest  the  edge 
meet  at  a  focus  nearer  the  lens  than  rays  that  are  nearer  the  centre. 
The  highest  degree  of  skill  is  required  in  the  optician  to  bring  the 
glass  to  such  a  shape  that  all  the  rays  having  the  same  direction 
shall  meet  at  the  same  point. 

Explain  1484.  Fig.  228  shows  how  the  magnifying  effect 
Fig.  228.  Of  a  double  convex  lens  is  produced.  The  insect 

Fig.  228. 


A  B  is  seen  by  the  light  reflected  from  it,  which  passes 
through  the  lens.  The  ray  from  A  to  (7,  instead  of  passing 
directly  on,  is  bent  in  accordance  with  the  laws  of  refrac- 
tion to  D ;  at  this  point  it  is  again  bent  downward,  so  that 
the  eye  which  receives  it  sees  this  portion  of  the  insect  in 
the  direction  D  a.  In  like  manner,  the  ray  of  light  from 
B  so  reaches  the  eye  us  to  appear  to  come  from  #.  Other 
rays  from  all  parts  of  the  insect  show  the  corresponding 
points  to  the  eye,  so  that  the  complete  insect  appears  to 
extend  from  a  to  I. 

Explain  1485.  Fig.  229  shows  how  the  rays  from  any 
Fig.  229.  "bright  object,  when  allowed  to  pass  through  the 
lens  and  fall  upon  a  flat  surface  properly  placed,  will  form 


434  MECHANICS. 

an  inverted  image  of  the  object.    Eays  from  the  point  of 
the  candle  flame  at  A,  diverging  to  different  points  on  the 

Fig.  239. 


lens,  are  converged  to  a  focus  at  a.  The  point  B,  in  like 
manner,  sends  forth  rays  that  find  a  focus  at  I ;  the  foci 
of  rays  from  other  points  of  the  flame  will  fall  in  their 
proper  places  between  a  and  #,  and  the  image  represents 
the  true  shape  of  the  flame. 

1486.  Iij  this  case  the  image  is  smaller  than  the  object^  but  it 
will  be  sufficiently  evident  upon  reflection,  from  the  method  by 
which  the  image  is  produced,  that  if  a  bright  object  were  placed  at 
a  b,  a  large  inverted  image  would  be  formed  at  A  B.  These  rela- 
tive positions  of  image  and  object  are  easily  found  for  any  good  lens 
by  experiment. 

Fig.  230. 


1487.  The  diminishing  effect  of  a  concave  lens  is  illus- 


MECHANICS. 


435 


Explain  trated  by  Fig.  230.  The  rays  of  light  from  top 
Fig.  230.  an(j  bottom  of  the  vase  A  I>,  when  passing  through 
the  lens,  are  rendered  less  convergent,  and  appear  when 
reaching  the  eye  to  proceed  from  a  much  smaller  object, 
as  a  b.  This  lens  cannot  be  made  to  form  an  image  on  a 
screen. 


Explain 
Fig.  231. 


1488.  The  use  of  the  convex  lens  of  the  eye  is 
in  Fig.  231.     The  rays  from  A  B,  after 


Fig.  221. 


233. 


passing  through  the  aqueous  humor,  the  crystalline  lens 
and  vitreous  humor  form  a  small  but  wonderfully  accurate 
image  (a  ~b)  on  the  retina. 

Explain  the  1489.  In  some  eyes  the  convex  lens  is  too 

defect  in  near-  far  from  the  retina,  and  consequently  the 
image  formed  is  very  indistinct.  For  such 
cases  spectacles  of  con- 
cave lenses  are  needed 
to  check  the  too  rapid 
convergence  of  the  ray:. 
People  needing  such 
aids  to  distinct  vision 
are  said  to  be  near- 
sighted. Fig.  232  represents  the  position  of  the  crystalline 
lens  in  such  cases. 


436 


MECHANICS. 


Ffc.  233. 


Explain        1490.   Fig.  233  represents  the  opposite  condi- 
Fig.23'3.  tion ;  here  no  distinct  image  is  formed,  because 

the  crystalline  lens  is 
too  near  the  retina. 
Tho  rays  require  to 
be  converged  more 
rapidly.  Spectacles 
of  convex  lenses  are 
therefore  required. 
People  needing  them  to  see  distinctly  are  called  far- 
sighted'. 

Fig.  234. 


Explain  the          1491.  The  Prism,  whose  construction  and 
°Prism.  properties  is  explained  on  page  252,  has  be- 

come of  late  one  of  the  most  important  optical  instruments. 


MECHANICS.  43? 

When  a  beam  of  light  falls  upon  the  prism,  as  repre- 
sented in  Fig.  234,  the  slowest  waves  are  deflected  from 
their  course  mucli  less  than  the  more  rapid  ones ;  hence 
what  would  be  a  bright  spot  on  the  wall,  if  the  prism  were 
removed,  becomes  by  the  different  amount  of  refraction  of 
different  waves  an  elongated  band  cf  several  colors,  called 
the  spectrum.  The  red  ray  is  refracted  least,  and  the 
violet  most  of  the  visible  rays.  Experiment  shows  that 
other  waves  pass  through  the  prism,  and  are  refracted  some 
less  than  the  red,  and  others  more  than  the  violet.  The 
waves  below  the  red  are  heat  waves,  and  may  be  detected 
by  the  thermometer ;  while  those  beyond  the  violet  pro- 
duce chemical  effects,  and  are  detected  by  such  sensitive 
preparations  as  the  photographer  uses. 

1493.  If  the  aperture  admitting  the  light  be  exceedingly  narrow, 
and  parallel  to  the  direction  of  the  prism,  dark '  lines  are  seen 
across  the  spectrum  lying  between  the  bands  of  color.  These  have 
long  been  known  as  the  Fraunhofer  lines,  and  are  accounted  for  as 
follows,  viz. :  Different  gases  and  vapors  intercept  certain  of  the 
waves  of  light,  and  permit  others  to  pass.  When  such  waves  are 
wanting  in  the  spectrum,  the  places  where  they  would  fall,  not 
being  illuminated  through  the  slit,  are  left  dark. 

The  instrument  used  in  the  study  of  these  lines  is  called  the 
Spectroscope.  The  spectrum  in  this  instrument,  instead  of  falling 
upon  a  screen,  is  directed  by  means  of  lenses  to  the  eye.  The 
lenses  serve  to  magnify  the  lines  and  the  spaces  between  them,  so 
that  a  far  greater  number  are  seen.  In  the  forms  of  the  spectro- 
scope designed  for  the  higher  scientific  uses,  several  prisms  are  em- 
ployed, and  also  measuring  scales  to  determine  the  exact  relative 
position.of  the  lines. 

On  what  facts         1493.   The  following  facts  form  the  foun- 
^ectrU^ek0   dation  of  spectroscopic  analysis  : 
founded?  (i)   Every  gas  or  vapor  possesses  the  property 

of  intercepting  certain  waves  of  light  while  permitting  others  to 
pass  freely  through. 

(2)  Each  gas  or  vapor  has  this  property  in  a  degree  peculiar  to 
itself,  so  that  the  dark  lines  of  the  spectrum  afford  a  means  for  its 
detection. 

(3)  Solid  or  liquid  luminous  bodies  give  spectra  without  dark 
lines  (continuous  spectra)  when  no  gas  or  vapor  is  in  the  path  of 
the  light. 


438 


MECHANICS. 


(4)  Luminous  gases  never  give  continuous 
spectra ;  they  yield  only  bright  lines  or  bands 
exactly  in  those  places  where  dark  lines  appear 
Vv-heii  the  gases  are  permitted  to  intercept  the 
light  from  a  luminous  solid  or  liquid  body. 

1494.  The  inference  drawn  from  these  facts 
respecting  sunlight  is,  that  the  sun  itself  is  a 
Bell-luminous  solid  or  liquid  body,  and  that  it  is 
surrounded  with   numerous  vapors,  which  we 
recognize   for  the  most  part  as   familiar  sub- 
stances.    Well-known  metals,  when  vaporized 
by  intense  heat,  can  be  made  to  produce  the 
lines  we  find  in  the  solar  spectrum. 

1495.  Fraunhofer  discovered  and  mapped  five 
hundred  or  six  hundred  of  these  lines  in  1814. 
As  many  as  six  thousand  are  now  observed  and 
so  located  by  philosophers  as  to  be  recognized. 
A  portion  of  these  lines  as  they  appear  in  the 
solar  spectrum,  with  the  letters  by  which  Fraun- 
hofer designated  the  larger  ones,  is  represented 
in  Fig.  235. 


&£££$  s^rswjffi2*«?fcS 

new  metals  not  previously  known. — The  knowl- 
edge of  the  existence  in  the  sun  of  substances 
familiar  to  us  as  forming  essential  portions  of 
the  earth. — That  the  fixed  stars  are  similar  to 
our  sun  in  constitution. — That  many  of  the  neb- 
ulas are  only  gaseous  bodies. 

1497.  In  addition  to  these  it  forms  by  far  the 
most  delicate  method  in  the  laboratory  for  the 
detection  of  the  chemical  elements. 

The  problems  at  which  philosophers  are  now 
engaged  with  this  wonderful  instrument  are  the 
determination  of  the  source  of  the  light  in  the 
Zodiacal  light,  the  Aurora  Borealis,  and  the 
Solar  Corona, — all  of  which  afford  one  or  two 
lines  not  familiar,  among  substances  of  the 
earth. 

Comets  hereafter  will  be  objects  of  the 
closest  scrutiny ;  and  when  bright  enough  to 
afford  a  spectrum,  the  question  of  their  consti- 
tution will  probably  be  satisfactorily  settled. 


Why  are  red, 


Colors  ? 


1498.  The  spectrum  is 
Senerally  Considered  to  be 
made  up  of  seven  different 


VIOLET. 


MECHANICS. 


430 


colors,  each  forming  a  separate  band.  It  is  found,  how- 
ever, that  having  pigments  of  red,  yellow,  and  blue  colors 
only,  all  other  colors  may  be  formed  by  mixing  these  in 
proper  proportions.  These  three  have  consequently  been 
called  the  Primary  Colors,  and  many  have  supposed  that 
the  remaining  colors  of  the  spectrum  were  caused  simply 
by  the  overlapping  of  the  bands  of  these  three  colors. 

What  is  said          1499.  On  the  other  hand,  it  is  found  that 

of  Secondary     the    so -called    Secondary    Colors  —  orange, 

Colors  ?  ,      .  ,   .  j    -i       i  i 

green,  and  violet  —  cannot  be  decomposed 

by  the  prism,  as  they  should  be  if  composed  only  of  mixed 
light. 

Explain  1500.  A  green  band  from  a  spectrum  being 
Fig.23Q.  pasS8(j  through  a  second  prism,  gives  the  same 


Fur.  230. 


Explain  the 
Eainlow. 


color  again.     (See  Fig.  236.)     The  same  result  is  obtained 
with  each  of  the  colors  of  the  spectrum. 

1501.  The  Rainbow  is  formed  by  the  re- 
fraction and  reflection  of  sunlight  in  the 
rain-drops — each  drop  acting  as  a  prism  to  decompose  the 
white  light. 

The  inner  or  primary  bow  is  the  brightest,  being  formed 
by  two  refractions  and  one  reflection,  while  the  secondary 
bow  is  formed  by  two  refractions  and  two  reflections.  As 
some  light  is  lost  at  each  reflection,  the  secondary  bow  is 
not  so  bright  as  the  primary. 


440 


MECHANICS. 


The  Fig.  237  represents  the  courses  of  the  rays  in  form- 
ing both  rainbows. 

Fig.  237. 


What  are  1502.  Colors  are  said  to  be  complementary 

Complementary  to  each  other  when,  taken  together,  they 
contain  all  the  constituents  of  white  light. 
This  recognizes  the  theory  of  three  primary  colors.  Red 
and  green  are  complementary,  because  green  may  be  com- 
posed of  yellow  and  blue.  Yellow  and  purple  are  comple- 
mentary to  each  other ;  also  blue  and  orange. 

Why  are  they  1503.  Complementary  colors  are  also  called 
called  Contrast-  .  ,  ,  ,  . , 

ing  Colors  f       contrasting  colors,  because,  when  seen  side 

by  side,  each  heightens  the  effect  of  the  other. 

Explain  1504.  Fig.  238  exhibits  the  pairs  of  contrasting 
Fig.  238.  colors.  The  primary  colors  are  joined  to  the  cen- 
tre by  heavy  lines ;  midway  between  these  are  the  second- 
ary colors,  so  placed  that  each  lies  between  the  primaries 
that  compose  it. 

Each  color  about  the  circle  is  complementary  to  the 
color  that  is  exactly  opposite.    The  colors  about  the  circle 


MECHANICS. 


441 


Fig.  238. 
GREEN 


ELLOWISH  GREEN 


VIOLET 


ORANGE 


RED 


having  double  names,  as  yellowish-green,  reddish-orange, 
etc.,  are  supposed  to  be 
composed  of  equal  parts 
of  the  colors  that  lie  ad- 
jacent. 

1505.  The  principles  gov- 
erning the  use  of  contrasting 
colors  are  of  great  import- 
ance to  decorators  and  all 
who  employ  colors  in  per- 
manent ornamentation.  Har- 
monies and  discords  are  rec-  tfioiET^ 
ognized  in  color  compositions 
as  well  as  in  music ;  but  we 
have  at  present  no  nomen- 
clature of  colors  which  will 
enable  us  to  identify  by  name 

any  but  the  simplest.  The  rules  to  be  observed  in  the  use  of  colors 
cannot,  therefore,  be  given,  until  names  are  devised  by  which,  they 
can  be  recognized. 

1506.  Color  Blindness  is  simply  inability 
distinguish  between  colors,  and  does  not 
imply  any  imperfection  in  the  eye,  so  far  as  seeing  form  is 
concerned.  To  a  person  entirely  color-blind,  all  objects 
appear  either  black  or  white  or  gray. 

1507.  People  partially  color-blind  usually  distinguish  yellow 
without  difficulty,  although  they  may  be  unable  to  see  any  differ- 
ence between  red  and  green.  In  an  examination  of  over  three  hun- 
dred persons  by  Dr.  Wilson,  of  Edinburgh,  one  in  every  fifty-five 
was  color-blind  to  this  extent.  A  slight  approach  to  color-blindness 
is  manifested  in  an  inability  to  see  the  slight  purple  tint  in  com- 
binations of  purple  and  blue.  This  is  very  common  indeed.  All 
attempts  to  cure  this  defect  in  the  eye  by  training  have  failed. 

Describe  the  1508.  TELESCOPES.— The  achromatic  tele- 

a^hromatic'  sc°Pe  described  in  Par-  90?  is  represented  in 
lens.  section  in  Fig.  239. 

It  will  be  observed  that  the  object-glass  is  composed  of 
two  different  lenses,  and  of  different  forms ;  the  halves  of 
the  lenses,  as,  for  instance,  the  parts  between  M  and  D, 
appear  in  the  section  like  two  prisms.  If  either  were  em- 


What  is  Color 
Blindness? 


442 


MECHANICS. 


ployed  alone,  it  would  act  like  a  prism,  and  refract  the 
colors  to  the  eye  somewhat  separated.    An  indistinct  view 

Fig.  239. 


is  the  result  of  such  action.  But  when  two  prisms  are  em- 
ployed, and  so  used  that  one  corrects  the  color  separation 
of  the  other,  the  opposite  surfaces  will  be  parallel,  and  the 
rays  leave  the  prisms  with  the  same  direction  in  whicli 
they  come  to  it,  provided  the  prisms  are  of  the  same  den- 
sity ;  otherwise  the  denser  prism  may  be  the  thinnest  one. 

Fig.  240. 


vr 


Describe  1509.  This  is  shown  in  Fig.  240.  The  ray  I, 
Fig.24Q.  under  the  action  of  the  prism  C,  alone  would 
form  a  spectrum  whose  violet  color  would  be  at  v,  and  red 
at  r ;  by  interposing  a  thinner  prism  of  material  having 
higher  dispersive  power,  the  ray  is  directed  to  v  r,  and  the 
colors  are  combined. 

1510.  In  the  case  of  the  object-glass  of  the  telescope,  the 
compound  lens  thus  formed  is  thickest  in  the  middle — 
hence  belongs  to  the  class  that  magnify.  The  rays  of  light 
from  the  distant  object  form  an  image  a  I  c  which  is 
viewed  with  the  eye-lens  P  Q. 


MECHANICS. 


443 


1511.  The  above  is  the  Astronomical  Telescope.  The 
arrangement  of  lenses  in  the  Terrestrial  Telescope  is  shown 
in  Fig.  241. 

Fig.  241. 


The  first  image  is  formed  at  n  m,  but  if  viewed  through 
the  lens  CD  would  appear  as  through  the  astronomical 
telescope,  inverted ;  hence  the  rays  are  allowed  to  cross 
each  other  at  L,  and  form  another  image  at  m'  n',  which 
being  seen  through  the"  eye-piece  C  H,  is  in  the  desired 
position. 

1512.  The  Galilean  Telescope  employs  only  one  object- 
glass  and  one  eye-glass  to  see  objects  erect.  This  arrange- 
ment forms  the  common  "opera  glass"  and  the  "mariner's 

Fig.  242. 


glass."  The  magnifying  power  is  low,  but  the  amount  of 
light  it  collects  and  brings  to  the  eye  adapts  it  for  use  at 
night. 

1513.  It  will  be  seen  by  the  figure  that  the  rays,  after 
passing  through  the  object-glass,  converge  in  such  a  man- 
ner as  would  form  an  image  at  m  n ;  but  the  concave  eye- 
19 


444  MECHANICS. 

piece  checks  this  convergence,  and  brings  the  rays  to  the 
eye  nearly  parallel. 

1514.  The  Beflecting  Telescope,  represented  in  section 
in  Fig.  141,  is  sometimes  constructed  so  as  to  be  used  by 
looking  in  at  the  side.    This  is  accomplished  by  setting 
the  reflector,  represented  at  C  in  Fig.  141,  at  such  an 
angle  that  the  collected  rays  are  directed  into  the  eye-piece 
in  the  side  of  the  tube. 

1515.  The  largest  Reflecting  Telescope  ever  constructed  is  that 
of  the  Earl  of  Rosse.     The  great  speculum  is  six  feet  in  diameter, 
and  the  tube  is  sixty  feet  long. 

The  largest  Refracting  Telescopes  yet  completed  have  object- 
glasses  of  only  twenty-three  inches  diameter. 


INDEX 


Aberration  of  light 384 

"  spherical  .    .  247 

\ccidental  colors   ...        .  252,  442 

,  Achromatic  . 247 

Acid,  carbonic 21 

Acid,    sulphuric,     effects    of    on 

water 187 

Acoustic  paradox 177 

Acoustics 173 

"         definition  of     .  18 

Acoustic  tubes 179 

Action 45 

Action  and  reaction,  illustration 

of 46 

Action,  suspension  of 85 

Actynolite «...    21 

Aeriform,  definition  of  .        ...    19 

«         fluids 138 

Aeriform  fluids   compressed    and 

expanded  without  limit    .    .    .  139 
Aoriform  fluids  have  no  cohesive 

attraction 139 

Aeriform  fluids  have  all  the  prop- 
erties of  liquids 140 

Aeriform  fluids  have  weight.  .    .  139 
Aeronaut,  how  he  descends  from 

a  balloon 38 

Aerolites 387 

A  flinity,  chemical 19,27 

Agents 18 

"      imponderable 18 

"      ponderable 18 

Air 140,  426 

Air,  a  bad  conductor  of  heat  .    .  191 

Air,  as  an  element 19 

Air-bladder  of  fishes  .......    47 

Air-chamber 163,  42C 

Air,  component  parts  of  the     .    .  140 
281  note 

Air,  compression   of,   caused    by 
gravity 39 


Air,  compressibility  of  the          .  162 

Air,  condensation  of  at  srrface  of 
the  earth 140 

Air,  condensed,  experiments  with  103 

Air  contained  in  wood  and  water, 
experiments  to  show  .....  161 

Air  diminishes  upwards  in  dens- 
ity      14C 

Air,  elasticity  of  the     ...  142,  102 

Air,  elasticity  of  the,  experiments 
showing 160 

Air,  effect  of  gravity  on  density  of  38 

Air  essential  to  animal  life,  ex- 
periment to  prove 16fc> 

Air  essential  to  combustion,  ex- 
periments to  prove lOf 

Air,  fluidity  of 142 

Air,  fluidity  of,  experiments  show- 
ing  1'io 

Air,  gravity  of  the,  experiments 
illustrating  ....  ...  1^7 

Air-gun 164 

Air,  how  a  mechanical  agent  .  .  142 
"  impenetrability  of  ...  22,  141 
««  inertia  of 28,  143 

Air,  inertia  of,  experiments  show- 
ing   1C5 

Air,  lightness  of  the 162 

"    materiality  of  the 162 

Air     miscellaneous    experiments 
Wlth  .    .    . 166 

Air  necessary  to  animal  life  and 
to  combustion 140 

Air,  of  wnat  composed 20 

Air,  pressure  of  the  as  the  depth  102 
**  pressure  of  in  all  directions  162 

Air,  pressure  of  the  on  a  barom- 
eter   HO 

Air,  pressure  of  the  on  a  square 
inch Hi 

Air,  pressure  of  the  on  the  body  141 

Air,  pressure  of  the  preserves  the 
liquid  form  of  some  bodies  .    .  1€9 


446 


INDEX. 


Air,  pressure  of  the  retards  ebul- 
lition  168 

Air-pump 154 

Air-pump,  experiments  performed 

by  the 157 

Air-pump  of  steam-engine    .    .    .  201 

Air-pump,  the  double 156 

Air,  resistance  of  the    .    .    .    .25,38 
Air,  resistance  of  the  to  a  cannon- 
ball   62 

Mr,  scales  for  weighing   .    .    .    .160 
Air,  two  principles,  properties  of  139 

'*    when  heaviest 140 

Air,  when   the  best  conductor  of 

sound 176 

Air,  why  not  visible 140 

Albite 20 

Alison,  extract  from 70 

•«  All's  well,"  how  far  heard    .    .  176 

Alumina 21 

Aluminum 20 

Ampere's  discoveries   in  electro- 
magnetism   309 

Ampere's  electro-magnetic  appa- 
ratus   314 

Analysis  of  the  motion  of  a  fall- 
ing body 52 

Angle 48 

Angles,  how  measured 48 

Angle  of  vision 219 

Angles  of  incidence  and  of  reflec- 
tion   48,  4(.i,  216 

Angles,  right,  obtuse  and  acute  .    48 

Animal  electricity 282 

Animals,  sagacity  of 92 

Annealing 31 

Antimony 20 

"         not  malleable 31 

Aphelion 349 

Apogee 349 

Apparatus    for     illustrating    the 
tendency  of  a  body  to   revolve 
around  its  shorter  axis  ....    61 
Apparition,  circle  of,  perpetual  .  385 

Apparitions,  deceptive 225 

Aqueous  humor 237,  239 

Arago's  experiments  on  velocity 

of  sound 176 

Arbor 81 

Archimedes'  boast  to  Hiero     .    .    95 
Archimedes,  burning  mirrors  of.  228 
Archimedes  discovers  the  method 
of  ascertaining  the  specific  grav- 
ity of  bodies 127  note 

Archimedes,  screw  of 132 

Arc  of  a  peiilulun         101 

4rctur\i«  .  .        .  309 


Aristotle's  opinion  of  the  verity 

of  a  falling  body 53 

Arsenic 20 

"       not  malleable 3? 

Asteroids 33S 

Astnea 339 

Astronomy,  definition  of  .  17,  18,  33£ 
Astronomers,  distinguished  .   .    .  330 

Astronomy,  father  of 336 

Atmosphere,  weight  of  the  .    .    .141 

Atmospheric  telegraph 331 

Attraction •  .  25,  26,  33 

capillary Ill 

chemical 27 

kinds  of 27 

law  of  falling  bodies  .    51 

mutual 34 

of  all  bodies 34 

of  cohesion '27 

««          of  gravitation  ....    27 

"          of  the  earth 33 

"          on  what  dependent  .  .    34 

Attwood's  machine 52 

Augite 21 

Austral  polarity 30 2 

Axes  of  the  planets,  inclination 

of 3->0 

Axis,  exact  sense  of 81 

Axis,   longer,   a   body   revolving 

around ••    .    .    .    t'l 

Axis  of  motion 59 

Axis  of  the  earth,  effects  of  its 

inclination 334 

Axis  of  the  earth,  geological  the- 
ory of 62 

Axis,  what  bodies  revolve  around 

an 59 

Axle.   .   .   .' 81 

Azote 20,  140 


B. 


Babbit's  metal 99 

Bain's  telegraph 326 

Baker,  the  Connecticut 191 

Balance-wheel 104 

Balance 75,  411 

Ballistic  pendulum 63 

.balloon,  how  to  descend  from  .  .  "58 
"  the  pneumatic  ....  161 
Ball,  thrown  in  a  horizontal  di 

rcction  ...........    64 

BailH,  force  of,  how  estimated  .  63 
Bands  with  one  and  two  centres 

of  motion 83 

Banks,  Sir  Joseph ,190 

Barber'?  Grammar  of  Elocution  180 


INDEX. 


44? 


Barium 20 

Barometer      .....  144  ind  note 
Barometer,  the  aneroid  or  porta- 
ble   145 

Barometer,  the  diagonal  .   .        .  145 
Barometer,  of  the  different  states 

01  the 148 

Barometer,  greatest  depression  of 

the 147 

barometer,  its  importance     146  note 
«*  rules  of  the  .  .    .    .    .  147 

"  the  mercurial  .   .    .    .145 

Base  of  a  body 67 

Batteries,  thermo-electric    .    .   .  335 

"        galvanic 287 

Battering  ram 105 

Battering  ram,  force  of,  how   es- 
timated   105 

Battery,  electrical 264 

Battery,  Grove's 293 

"        how  discharged  silently  2G5 
Battery  of    the  electro-magnetic 

telegraph 321 

Battery,    protected    sulphate    of 

copper 293 

Battery,  Sraee's 290 

"        sulphate  of  copper    .   .  292 

Beam  of  light . 213 

Belgrade,  battle  of,  and  the  cornet  380 
Bellows,   hydrostatic,    how    con- 
structed     119 

Boll,  the  diver's  or  the  diving    .  150 

Bevelled  wheels 85 

Birds,  bodies  of 123 

•«      how  they  fly 47 

•*      muscular  power  of  ....    47 

Bismuth 20 

««        not  malleable .        ...    31 

Bissextile,  meaning  of 396 

Black 252 

Black  lead,  uses   of  in  overcom- 
ing friction 99 

Bladder-glass 159 

Bladder,  inflated,  why  compress- 
ed in  water 115 

Boats,  how  propelled 47 

Boats,   on   what    principle    they 

float 123 

Boats,  motion  in,  why  impercep- 
tible  26 

Bode's  law 342 

Bodies 18 

"       attraction  of 33 

Bodies  of  drowned  persons,  why 

they  sink  and  afterwards  rise  .  123 

Bodies,  what  are  easily  overset  .    69 

"       what  stand  most  firmly  .    68 


Bodies,  what  will  rise   aud  what 

will  fall  in  air 4C 

Body  acted  upon  by  three  or  more 

forces 57 

Body,  parts  of  which  move  with 

greatest  velocity 60 

Bodies,  what  ones  will  float  and 

what  sink  in  water 123 

Body,  when  it  will  fall 66 

Bohemia  slate,  formations  of  .  .  23 

Bolt-head,  and  jar 167 

Bomine  M 370 

Bones  of  a  man's  arm,  levers  of 

third  kind 77 

Borax .  20 

Boreal  polarity 302 

Bottle,  effect  of  pressure  of  the 

sea  upon 115 

Boyle 144 

Boynton's,  Dr.,  chart  of  materi- 
als which  form  granite  ....  21 
Bramah's  hydrostatic  press  .  .  121 
Brass,  how  made  brittle  ....  30 

Breadth 23 

Breaoc-wheel 8 '2,  83 

Brittleness 27,  30 

Brittleness,  how  acquired  by  iron, 

steel,  copper  and  brass  ....  30 

Bromine 2(. 

Brooks,  how  formed 124 

Buckets  of  water-wheels  ....  82 
Buckets  of  water,  why  heavier 

when  lifted  from  the  well  .  .  12fc 
Bulk  of  a  body,  how  ascertained 

from  its  weight 125 

Burdens,  how  made  unequal  .  .  77 
Burning-glasses 228,  23-'» 


C. 


Cadmium 20 

Calcium 20 

Calliope 339 

Caloric 187,427 

Calorimotor 297 

Camera  obscura 219,  240 

Camera   obscura,    portable,   how 

made 21y 

Cannon-ball,     greatest    velocity 

that  can  be  given  to 03 

Cannon-ball,  force  of  the  resist- 
ance of  the  air  to 62 

Cannon,  how  far  heard 17<! 

Caoutchouc,  or  India-rubber    .    .    bU 
"  balls,  elasticity  of  .    47 

Capillary  attraction Ill 

««  "          cause  of  .    .111 


448 


INDEX, 


CapUlarj  tubes Ill  | 

Capstan ,        .80 

Capstan  and  windlass,  diffeionoe 

between .    .    80 

Carbon 20 

Carbonate  of  lime 21 

Carbonate  of  magnesia 21 

Carbonic  acid  .   .    ! 21 

Carriages,  high,  why  dangerous  .    68 

Carronades 63 

Cartesian  devil 162 

Cask,  how  burst  by   hydrostatic 

pressure 1'20  note 

Cassegranian  telescope      ....  250 
Castors,  why   applied   to   legs  of 

tables,  <tc 85 

Catoptrics 215 

Celestial  bodies,  true  place  of    .  384 
Celsius'  thermometer    .....  149 

Central  forces 59 

Centre  of  gravity  .    .    57,  58,  59,  66 
Centre   of    gravity,    illustrations 

of 66  note,  409 

Centre  of  magnitude     .    .  58,  59,  66 
Centre  of  motion    .    .    .    .  58,  5y,  71 

Centre  of  sphericity 37 

Centre,     what      bodies      revolve 

around  a 59 

Centres 58 

Centrifugal  force 59 

Centrifugal  force,  eifect  of  on  a 

body  revolving  around  its  longer 

axis 61 

Centrifugal  force,  to  what  propor- 
tioned   60 

Centrifugal  force,  where  greatest  103 
"  meaning  of     ....    59 

Centripetal  force 59 

"          meaning  of     ....    59 

Ceres 339 

Cerium 20 

Chain-pump 131 

Chaises,  tops  of,  toggle-joint   .        97 

Chamfered 91 

Chantrey,  the  sculptor  .....  191 

Charged,  meaning  of 261 

"  Charlemagne,"    experiment    on 

board  of  the     ........  115 

Charles  V.  and  the  comet    .    .    .  379 
Charles'  wain  or  wagon    ....  398 

Chart  of  materials    forming   the 

crust  of  the  earth      20 

Chemical  affinity 19,  27 

Chemical  attraction 27  j 

Chemical  effects  of  light  .    .  256,  257 

Chemical  electricity 259 

Chemistry  19,  110 


Chimneys,  glass,  Low    preserved 

from  cracking >  -.»? 

Chisels,   on    what    principle   coii 

structed        91 

Chlorine 20 

Chlorite 21 

Chord,  musical,  how  produced    .  182 

Choroid 237,  240 

Chromatics 251,  436 

Chromium 20 

Circle 48 

Circle  of  perpetual  apparition     .  385 

Circies 59 

Circles,  circumference  of,  how  di- 
vided      48,  365 

Circular  motion 5S* 

Circular  motion  changed  to  rec- 
tilinear by  cranks 81 

Circular  motion,  how  caused    .    .    58 


Clai 


21 


Climates,  cause  of 354 

Clock,  before  and  after  the  sun  .  397 

"      how  regulated 102 

"  moving  power  of  ....  104 
Clock,  periods  when  it  agrees  with 

the  sun 397 

Clocks,   why  they  go    fastest   in 

winter 103 

Clock,  what  it  is 102 

"  wheels  of,  their  use  ...  102 
Clothing,  cause  of  warmth  of  .  .  189 
Clouds 24 

"  of  what  composed  ....  186 
Cobalt 20,  298 

"      not  malleable 31 

Coffee-pots,  why  with  wooden  han- 
dles  ..............  190 

Cogs 83,  84 

Cohesion,  attraction  of 27 

Cohesion,  attraction  of,  its  effects 

on  watery  particles 186 

Cold 185, 192 

Cold,  its  effects  on  the  density  of 

bodies 192 

Colors 254 

"      accidental 252 

Columbium 20 

Comets 372 

"      density  of 379 

Comet,  Halloy's,  as  seen  by  Sir 

John  Herschel,  and  by  Struve. 

377,378,379 
Comet,  Halley's,  periodical  time 

of 371 

Comets,  how  regarded  former  y  .  373 
Comets  in  the'  solar  system,  num- 

b«r  of    .  .379 


INDEX. 


449 


Comets,  Kepler's  opinion  of  their        , 

number 380  ; 

Comets,  number  of     ......  373  j 

Comet  of  1080 375  I 

"       "  1744 376  ! 

"       "  1811 373 

Comet   of  1853,  Mr.  Hind's   ac- 
count of  the     . 381 

Comet  of  1856 379 

Comets,  orbits  of .374 

Comets,  return  of,  first  predicted 
by  Halley,  Encke,  and  liiela  .  377 

Cornets,  tails  of 374 

"       velocity  of 37  ~ 

Common  centre  of  gravity  of  two 

or  more  bodies 69 

Complex  wheel-work 83  j 

Compound  battery 290 

"          lever 75 

"          motion 55 

"  "      how  produced  .    54 

Compressibility 27,  28,  29 

Concave  mirrors 222 

"             "      effects  of    ...  225 
Concave  mirrors,   laws  of  reflec- 
tion from 227 

Concave  mirrors,   peculiar  prop- 
erty of 224 

Concave  mirror,  true  focus  of  .    .  224 

"        screw 94 

Concave  surfaces,  facts  with  re- 
gard to     . 236 

Condensation 140 

Condensed 140 

Condenser 1(J8 

"  of  steam-engine  .  .  .  200 
Condensing  syringe  ....  156, 103 
Conduction  of  heat  .  .  .  .190,428 
Conductors  of  the  galvanic  fluid  .  285 

258,  260 

"          of  heat 189 

Cone      69 

Conic  sections 341 

Conjunction,   inferior   and    supe- 
rior    349 

Connecticut  baker 191 

Conservatory  of  arts  and  trades, 
how  restored  to  perpendicular  .193 

Constellations SS3 

"  of  the  zodiac     .    .  347 

Oontractibility 28 

Converging  rays 212,  227 

Conversation    in     polar     regions 
heard  at  great  iistances  .    .    .  176 

Convex  mirrors 222 

Convex  mirrors,  laws  of  reflection 
from  .    .  220 


Convex  mirrors,  effects  of     .        .  224 

Convex  screw 94 

Conv-ex  surfaces,  facts  with  regard 

to ^35 

Copernicus 33  G 

Copper 20 

Copper  and  tin,  sonorous  proper- 
ties of 30 

Copper,  how  made  brittle     ...    30 

Cords,  tenacity  of 32 

Cork,  how  deep  it  will  sink  ...  123 
"     why  lighter  than  lead     .    .    34 

Cornea  237,  238 

Corpuscular  theory  of  light  .    .    .211 

Couronne  des  tasses 290 

Crank,  dead  point  of 81 

Cranks      80 

Crown-wheel    . 84 

Crust  of  the  earth,  materials  com- 
posing the 20 

Crystalline  humor,  convexity,  how 

increased  or  diminished    .    .    .  241 
Crystalline  humor,  effect  of  when 

too  round 242 

Crystalline  lens 237,  435 

Cup  of  Tantalus 133 

Cups,  the  Magdeburgh 157 

Current   velocity  of  a,  how  meas- 
ured   130 

Curve  of  a  projectile,  on  what  de- 
pendent  64 

Curvilinear  motion 61 

Cutting  instruments 91 

Cylinder,  definition  of  a  ....    79 
Cylinder,  how  made  to  roll  up  a 

slope 68 

Cylinder,  wheel  substituted  for  .    79 


1). 


Daguerreotype  proofs 257 

Darkness  produced  by  two   rays 

of  light 212  note 

Davies'  Treatise  on  Magnetism  .316 
Day  and  night,  cause  of  ....  358 
Days  and  nights,  cause  of  differ- 
ence in  length  of   ......  350 

Dead  point  of  a  crank 81 

Delisle's  thermometer 149 

Delphi,  oracle  of 180 

Demetrius  Poliorcetes  .    .        .    .  105 

Density 27,28 

Density  of  air,  effect   of  gravity    . 

on. 38 

Depth  of  a  well,  how  estimated  .    53 
De.?caitcs         H* 


450 


INL>EX. 


Devil,  the  Cartesian 162 

Dew  and  fog,  difference  between.  150 

Dew,  how  produced 150 

Diagonal 48 

"        of  a  parallelogram    .    .    55 

"        of  a  square 55 

Diallage 21 

Diameter 48 

Diameter,  equatorial,  how  length- 
ened   61 

Diameter,  equatorial  of  the  earth, 

longer  than  polar,  and  why  .    .    61 
Diameter  of  the  earth,  equatorial 

IUM  polar 102 

Diameter  of  the  earth,  how  ascer- 
tained    365 

Didyniuin 20 

Digits 31)5 

Dilatability 29 

Dionysius,  ear  of 178 

Dionysius,  how  he  overheard  his 

prisoners 178 

Dioptrics 230 

"         laws  of 230 

Dipping  of  a  magnet 303 

Dipping  of  a  magnet,  how  reme- 
died   303 

Direction 41 

"        line  of 66 

Discharge,  the  jointed 264 

Dissolving  views 246 

Distance  at  which   a  man   is  in- 
visible   220 

Distance,   greatest  which  can  be 

estimated 382 

Distances    measured  by  veU<;ty 

of  sound  . 177 

Distillation 194 

Distilled  water,  why  used  as  stand- 
ard of  specific  gravity  .    .    .    .  123 

Diverging  rays 212 

Divers,  limit  to  the  depth  of  115,426 
Diving  bell,  or  diver's  bell  .  .  .  160 

Divisibility 21 

"          extent  of 23 

«  definition  of  ....    23 

"  Dodge,"  how  children  .  .  .  .  26 
Double  action  of  the  steam-engine  200 
Drowned  persons,  why  they  sink 

and  afterwards  rise 123 

Ductility.    .  • 27,31 

Dynamics 17 

"        meaning  of 18 


R. 


Earth      .  . 


363 


Earth,  n  good  conductor  of  8<mcd  i'<9 

"     as  viewed  from  the  moon  .  36-1 

"     attraction  of  the     .    .    .    .    33 

"     centre  of  gravity  of       .        37 

Earth,   consequences    of  a   wore 

rapid  rotation  of  the 366 

Earth,  constituent  elements  of  tho  20 
Earth,  crust  of  the,  mate  vials  com- 
posing   .    .    20 

Earth,  diameter   of,   hrw   ascer- 
tained   365 

Earth,  figure  of  the 364 

"      how  known  to  be  -ound    .  364 
Earth,  how  much  larger  than  any 

falling  body 33 

Earth,  motions  of  its  inhabitants  365 
"      nearer  the  sun  in  whiter  .  352 
Earth,  parts  of  which  move  most 

rapidly 61 

Earth,  strata  of  the 20 

Earth,  the   principal  reservoir  of 

electricity 261 

Ebullition   retarded   by  pre^uro 

of  the  air 168 

Echo 177 

"    why  never  heard  at  set.      .  178 

Eclipse 392 

"      annular 394 

Eclipses,  greatest  number  of  ir  \ 

year 396 

Eclipse,  lunar,  to  whom  visible  .  395 
"        solar,  to  whom  visible    .  395 
"        total  of  1806     .    .    .    .    .396 

Eclipses,   why  more   of  the   sun 
than  of  the  moon    ......  303 

Eclipse,  why  not  at  every  new  and 

full  moon 392 

Eclipse,  partial 394 

total 394 

Ecliptic 345 

Egeria  . 339 

Ehrenberg's  microscopic  observa- 
tions   23 

Elastic  fluids 139 

Elasticity 27,  29,  30 

of  air 142 

of  gaseous  bodies     .    .    30 

of  ivory 46 

Electrical  battery 264 

bells 273 

fire-alarm 330 

machine     ...     ...  266 

Electrical  machine,   experiments 
with 270 

1  Electrical  sportsman 275 
Electric  current,  direction  of j  iiuw 
I      ascertained .   .              .        .      :P# 


INDEA. 


451 


Electrical  tellurium 272 

Electric  Huid,  velocity  of  ....    43 

Electricity 17,  18,  258 

Electricity  acquired  by  induction  278 
Electricity  and  magnetism,  resem- 
blance between 302 

Electricitv,  animal 282 

"  by  induction  ....  2GG 

*'          circuit  of 265 

Electricity  as  excited  by  galvan-    . 
isui  and  by  friction,  difference 

between 283 

Electricity  by  transfer 2(36 

Electricity,     effects     of     similar 

states    2G3 

Electricity,  frictional    .    .    .  282,  283 
Electricity,  frictional  and  chemi- 
cal, how  they  differ    294 

Electricity,  galvanic,  quantity  of.295 

"  nature  of 259 

Electricity,  quantity  of  excited  by 

chemical  action     ....  284  note 
Electricity,  simplest  mode  of  ex- 
citing     2G2 

Electricity,  the  vitreous  or  posi- 
tive, the  resinous  or  negative  .  2G2 
Electricity,  three  states  of  ...  335 

"          voltaic 283 

Electrics. 258,  2GO 

Electric  telegraph,  history  of  the  329 

Electro-magnet 317 

Electro-magnet,  communication 
of  magnetism  to  steel  by  means 

of 318 

Electro-magnetic  multiplier     .    .  313 
Electro-magnetism     .    .   17,  2GO,  308 
Electo-magnet,    the  U  or  horse- 
shoe   319 

Electro-magnetism,  definition  of  .   18 
Electro-magnetism,  discoveries  of 
(Ersted,  Faraday,  Ampere,  Ara- 
go,  and  Sir  11.  Davy      .    .  308,  309 
Electro-magnetism,  facts  of     .    .  309 
Electro-magnetic  induction      .    .312 
Electro-magnetic  rotation  .  313,  31G 
note 

Electro-magnetic  telegraph,  sig- 
nal-key and  registering  appa- 
ratus of  the 322 

Electro-magnet    of  Prof.    Henry 

and  Dr.  Ten  Eyck  ....  317 
Electro-magnetic  telegraph  .  .  319 
Electro-magnetic  telegraph,  how 

put  into  operation 324 

Electru-mutaliurgy         331 

Electrometer 2G8 

Wectrophoru? 2(i(J 

19* 


Electro-plastic  process 331 

Electro  plating  and  gilding  .    .    .  331 

Electroscope 2b9 

Electrotype  process   231 

Elementary  substances,  enumera- 
tion of 20 

Elements,  the  four 19 

Ellipse      341 

Elocution,  Barber's  Grammar  of  .  180 
"  Empty,"  common  meaning  of  .    OS 

Endosmose 27,  112 

Engineer,  how  enabled  to   direct 

his  guns 65 

Engine,  the  fire 154 

the  steam 196 

Equilibrium 74,  75 

"  of  fluids 110 

Equilibrium  of  fluids,  exemplified 

by  means  of  the  siphon     .    .    .  133 
Equilibrium  of  fluids,  now  disturb- 
ed by  waves 131 

Equilibrium  of  fluids  of  different 

densities 113 

Equilibrium    of  mercury,   water, 

oil,  air,  Ac 113 

Equinoxes   358 

"  precession  of  the  .  .  .  397 
Equivalent,  mechanical  .  .  53,  431 
Erect,  why  objects  are  seen  .  .  .  2J1 

Erbium 20 

Escapement-wheel 104 

Essential  property,  meaning  of  .    21 
Essential  properties  of  matter      .    21 

Eunomia 339 

Evaporation,  Dr.  Watson's  exper- 
iment     150 

Eye 237,  4Vi 

"    a  camera  obscura 240 

"    different  parts  of  the     .    .    .237 

Eye-glass 248 

Ejre,  imperfections  of,  how  caused  242 

"    of  what  composed 237 

Eyes,  two,  why  they  do  not  cause 

double  vision 241 

Exercises  for  solution 53 

Exhausting  syringe 163 

Exosmose 27,  112 

Expansibility      27,  29 

Expansion,    bow    it   differs    from 

dilatation 2? 

Experiments    showing    inert:r.    ~f 

air 165 

Extension 21,  23 


F. 

Fahrenheit's  thermometer 


148 


452 


INDEX. 


Fall'.ng  bodies,  law  of 51 

Faraday,  announcement  of  in  re- 
lation to  solar  spots  and  mag- 
netic variation 304 

Faraday's  discoveries  in  electro- 
magnetism  308 

Faraday's  electro-magnetic  appa- 
ratus   313 

Faraday \  nomenclature  of  elec- 
tricity   259 

February,   why    29    days    every 

fourth  year 397 

Feldspar 21 

Fire-alarm,  the  electrical     .    .    .330 

Fire,  as  an  element 19 

Fire-engine      154 

Fifth 184 

"     how  produced 182 

Figure 21,  23 

Fishes,  how   thev   swim,   rise   or 

sink,  &c 47 

Fixed  pulley,  mechanical  advan- 
tage of 8V 

Fixed  pulley,  operation  of  the    .    87 
Flavio  de  Melfi,  inventor  of  mari- 
ner's Ccrapass 306 

Flexibility 27,  31 

Float,  how   heavy  bodies  can  be 

made  to 38 

Float-boards  of  water-wheels  .    .    82 

Flora .339 

Florence,  experiment  made  at  on 

impenetrability  of  water  .  22,  109 
Fluid  and  solid  bodies,  diiference 

between 108 

Fluid,  definition  of 108 

Fluidity  of  air 142,105 

"        what  constitutes     ...  108 

Fluid  pressure,  law  of 115 

Fluids,  aeriform 138 

Fluids,   aeriform,  expanded   and 

compressed  without  limit  .  .  139 
Fluids  and  liquids,  how  different  109 
Fluids,  effects  of  their  peculiar 

gravitation 113 

Fluids,  equilibrium  of  ...  122, 133 
Fluids,    downward     pressure     of, 

how  shown 114 

Plaids,  gravitation  of  .    .    .    .    .110 

«        how  different  from  liquids  109 

'        how  they  gravitate    .    .    .113 

"      lateral  pressure  of  .  114,  116 

117 

Fluids  level  or  equilibrium  of   .110 
••      mechanical  agency  of  .    .138 
Fluids  of  dine  rent  densities,  gi  av- 
'uitiun  of  ....  1 12 


Fluids,  particles  of,  how  arranged  114 

"       pressure  of IK 

Fluids,  pressure  of,  according  to 

height 119,120 

Fluids,  pressure  of,  on  what  de- 
pendent     118 

Fluids,  pressure  of,  to  what  pro- 
portional       115 

Fluids,  surface  of 110 

Fluids,  upward  pressure  of  .  114, 117 
Fluids,  why  unsusceptible  of  foim- 
ation  into  figures    .    .   ,   .    .    .110 

Fluorine 20 

Fly 143 

Flying  of  birds,  how  effecsted  .    .    4V 

Fly-wheels «i| 

Fly-wheels  and  the  dead  points  ^f 

cranks Jl 

Fly-wheel  in  the  steam-engine  .  *03 
Focus  of  concave  mirrors  .  .  .  224 
Fog  and  dew,  difference  between  150 

Fog,  how  produced 150 

Force 41 

Forces,  at  an  angle    .       ...        58 
"      effects  of    .    .  .    .    55 

*'      three  or  more  in  action  .    57 
"      unequal  at  right  angles  .    56 

Forcing-pump 153 

Formulae 44 

Fortuna 33U 

Fountain,  glass  and  jet         .    .    .  159 

Fountain,  Hero's 138 

Fountains,    artificial,    how    con- 
structed     137 

Fountains,  how  formed     .    .    .    .137 

Fourth .184 

Fowling-pieces,  length  of     ...    63 
Franklin,  inventor  of   lightning- 
rods 281 

Free  heat     . 187 

Frictional  electricity    .    .    .  259,  283 

Friction 90  note,  98 

"  cause  of  ...  c  ...  99 
**  how  diminished  .  .  -  .  99 
"  boAV  increased  .....  99 
"  loss  of  power  caused  by  .  Us? 
"  important  uses  of  ...  100 
Friction  of  the  beds  and  banks  of 

rivers 130 

Friction,  particles  of  fluids  desti- 
tute of  ...    ^ 108 

Friction-wheels 99 

Fuel,  combustion  of 24 

I  Fulcrum 70,  71,  T'-s 

**       generally  a  pin  >>r  a  rivet  1>J 
|  Fulcrum    in    levers   of    different 
kinds     .  71 


Pxilorum  of  steelyard; 74 

Pulton,  Robert 200 

Fundamental  law  of  mechanics    .    71 
Fusee  of  a  watch     .  .  107 


Galaxy 383 

Galileo 100,  143,  337 

Galileo's  experiment  at  Pisa   to 

prove  his  law  of  falling  bodies  52 
Galileo's  law  of  falling  bodies  .  52 
Galvanic  action,  three  elements 

necessary  for 285 

Galvanic  batteries 287 

"         battery 289 

«         circle 286 

Galvanic  circle,  effects  of,  how  in- 
creased  287 

Galvanic  circle,  essential  parts  of 

a 286 

Galvanic  circle,  simplest,  of  what 

composed 286 

Galvanic  electricity 259 

Galvanic   electricity,  process   for 

obtaining 286 

Galvanic  fluid,  how  excited     .    .  284 

piles 287 

Galvanism  . 17,18,283 

"         fact?  explained  by   .    .  296 
Galvano-phustie  process     ....  331 

Galvanotype 331 

GarrnentSj  light-colored  why  cool  191 
"  linen,  why  cool    .    .    .  189 

Garments,  to  what  they  owe  their 

strength 100 

Garments,  woollen,  why  warm     .  189 

Garnet 21 

Gaseous  bodies,  elasticity  of   .    .    30 
Gaseous   bodies,  to  what   degree 

they  may  be  dilated 29 

Gases 139 

Gases,  how  prevented  from  rising 

from  a  fl'-iid 168 

Gay  Lussa"'s  experiments  on  the 

velocitj  /f  sound 176 

Gearing        83 

Geology 62 

Georgiurv  Bidas 368 

Gibbous 388 

Glucinuia 20 

Gold 20 

G«  Id,  both  ductile  and  malleable    31 

divisibility  of 23 

Gclit.  the   most  malleable  of  all 

metals 31 

m&t  \  it?  l.rittleuese      .        ...    32 


Gla^x,  the  bladder l:<4 

"     the  fountain  anJ  jet  .    .      169 

"     the  hand ,158 

"  the  India-rubber  .  .  .  159 
Glass,  wh^  easily  cracked  whea 

suddenly  heated 1 M 

Glass,  why  used  in  mirrors  .    .    .221 

Governor 106.  2\>0 

Governor  applit  1  to  steam-engine 

by  James  Watt 106,  203 

Governor,  explanation  of  the  .    .  106 

"         uses  of  the 106 

Grain  of  hammered  gold  ....    23 
Grand  law  of  nature   .    ...  69  note 

Granite 20 

Gravitation,  attraction  of    ...    27 
of  fluids     ...  110,112 

Gravity 25,  33 

Gravity  causes  pressure  of  fluids 

upward  as  well  as  down  U7,  418 
Gravity,  centre  of  ....  37,  59,  66 
Gravity,  effect  of  on  density  of  air  38 
Gravity,  effects  of  on  different 

bodies  , U 

Gravity,  force  of,  not  affected  bv 

projection t',4 

G rarity,  force  of  on  projectiles  .     *'>2 
"             "        where  greatest    35 
"       how  it  increases  and  de- 
creases       35 

Gravity,  law  of  terrestrial    .    .    .    35 
Gravity,  specific      ...  40,  126,  41ft 
Gravity,  specific,  scales  for  ascer- 
taining      126 

Gravity,  specific,  standard  of  123, 416 

"       terrestrial    34 

Great  Bear 398 

Green  sand 21 

Gregorian  telescope 2")0 

Gridiron  pendulums 103 

(j  rove's  battery  ........  2iJ3 

Gudgeons 8C 

Guericke,  Otto 158 

Guinea  and  feather  drop  ....  165 

Gunnery,  science  of 62 

Gunpowder,  force  of 63 

Gunpowder,  great  charges  of  use- 
less and  dangerous 63 

Guns,  how  tested    .        .....    (u1 

Guns,  short  ones,  why  preferable    63 

Gun,  the  air !*>' 

ijymnotuselectricus  .....      28; 

H. 

Hail,  how  formed 124, 16V 

•*     Low  it  differs  from  snow    .  lt^ 


154 


INDEX. 


Hair -spring 104 

flail,  Captain  Basil 146 

Bblley's   comet    as   seen    oy  Sir 

JohnHerschel 379 

Hand-glass 158 

Handles  of  tea-pots,  «tc.    why  of 

wood .    .  190 

Hare's  caloriinotor 297 

Harmony 181 

"         how  produced    ....  183 
Harmony,   sciance    of,   on    what 

founded 182 

Harvest-moon 389 

Heat  accompanies  all  great  chang- 
es in  bodies 110 

Heat,  application  of  its  expansive 
power  as  a  mechanical  agent  193, 431 

Heat  and  cold 187,  4<?8 

"     conductors  of 189 

"     effects  of 188,  429 

"     effects  of  on  bodies  .    .  185, 188 
Heat,  effects  of  on  density  of  sub- 
stances   " 192 

Heat,  effects  of  on  water  .    .  186,194 

Heat,  free 187 

««      first  law  of 189 

"  imperfect  conductors  of  .  190 
•«.  its  effects  on  a  body  ...  141 
*'  most  obvious  effects  of  .  .  193 

•«      how  propagated 190 

"      latent 187,  429 

"      law  of  the  reflection  of  .    .  191 

"      laws  of 185 

"      nature  of 185,  427 

•«      of  the  sun 188 

Heat  produced  by  electrical   ac- 
tion   188 

Heat,  sources  of 187 

Hearing  trumpets 178 

Heavenly  bodies,  motion   of  the 

when  the  most  rapid 350 

Heavenly  bodies,  why  not  seen  in 

their  true  place 232 

Heavens,  why  brigh.  in  the  day- 
time   218 

Hebe 339 

Height  of  a  building,  how  esti- 
mated    53 

Height  to  which  a  body  projected 

upward  will  rise 54 

Heliacal  ring 318 

Heliography 257 

Helix 316 

tlenry's  and  Dr.  Ten  JSyck's  elec- 
tro-magnet   317 

138 


Ilerschel    sees    stars    through   a 

comet 37V 

Ilerschel,  Sir  J.  F.  W.'B  illustra- 
tion of  the  size  and  distance  of 

the  planets 344 

Ilerschel,  Sir  John's  opinion   of 

the  height  of  the  atmosphere  .    38 
Herschel's  telescope  and  its  pow- 
er    251,337 

Heterogeneous Ii> 

Hiero  employs  Archimedes  to  de- 
tect the  adulteration  of  a  crown. 127 
Hind's  account  of  the  comet  of 

1853 381 

Hipparchus,  father  of  astronomy  .  336 

Homogeneous 19 

Hornblende. 21 

Horizontal  motion  docj  not  affect 

that  of  gravity 65 

Horse-power  as   applied    to    the 

steam-engine,  meaning  of    .    .  199 
Horses,  how  made  to  draw  unequal 

portions  of  a  load 77 

House's  printing  te'egraph  .    .    .  328 

Human  voice,  powers  of  the     .    .  180 

Humor,  the  vitreous  ....  237,  259 

"       the  aqueous  .    .    .    .237,239 

Hunter's  moon 383 

"        screw 95 

Hydraulics  .    .     17, 18,  108,  128,  418 

Hydraulic-ram 133 

Hydrodynamics 108,129 

Hydro-electric     .    . 334 

Hydrogen '20 

"        gas  generator   ....  275 

"         pistol 274 

Hydrometer 128,417 

Hydrostatic    bellows,    how    con- 
structed     119 

Hydrostatic  paradox 118 

Hydrostatic  press,  Bramah's  121,  415 
Hydrostatic   pressure,  as   a   me- 
chanical power 121 

Hydrostatic  pressure,  caused  by 

height,  not  by  quantity    .    .    .119 
Hydrostatics    ...     17,  18,  108,  412 

Hygeia 339 

Hygrometer 149, 150 

Hyperbola 3-11 

Hypcrsthene 21 


I. 


Ice  formed  under  a  receiver    .      1C9 
"  Low  mode  to  melt  rapidly  .    .  1W1 


INDEX. 


455 


l(»e,  why  wrapped  in  woollen  or        i 

packed  in  shavings 190  I 

foe,  why  wooden  spoons  and  forks 

are  used  for 190 

[mage  from  concave  mirrors    .    .  225 
"         '«    convex  mirrors     .    .  '223 

"      inverted 218 

Impenetrability 21,  22 

Imponderable  agents     ....         18 

Incidence,  angle  of 48 

Incident  motion .    .......    47 

Incident  rays 216 

Inclination  of  earth's  axis,  effects 

of 354 

Inclined  plans 90 

*«  "  advantage  of  .  .  91 
"  "  application  of  the  91 
"  "  principle  of  the  .  90 

Incombustible  bodies 188 

Indestructibility 21,  23 

India  rubber 30 

"         "       balls,  elasticity  of  .    47 

"         "       glass 158 

fndustioa,  electricity  by  .    .  266,  278 

"          electro-iaagnetic     .    .312 

Inertia     ......    21,  24,  26,  41 

"      experiment  to  illustrate  .    25 

"      of  air 38,  14'3,  165 

**      of  a  fluid,  effects  of  the  .  134 
"      of  fly-wheels  ......    81 

"      of  water 98 

Inferior  conjunction 349 

"       planets 343 

Infusoria 23 

Instruments  for  raising  water  .  .131 
Insulated,  meaning  of  ...  201,  270 
Intensity  as  applied  to  electricity, 

meaning  of 295 

'"  In  vacuo" 98 

"ridium 20 

Iodine 20 

Irene 3-3'J 

Iris  of  the  eye  .    .    .    .    .    .       237,238 

Iris,  the  planet  or  ast eroid   .        .    .339 

Iron 20 

Iron,  a  knowledge  of  the  uses  of 
the  first  step  towards  civiliza- 
tion   31 

too.  ductile   but   not   malleable 

into  thin  plates 31 

i.  n\t  how  made  brittle 30 

"     oxide  of 21 

"     when  most  malleable  ...    31 
,  elasticity  rf 30,  46  ! 


Jansen ...      33* 

Jerusalem,  siege  of   .    .        .    .      106 
Jet,  the  straight  and  revolving     163 

Jointed  discharger 2»J1 

Juno 339 

Jupiter .367,368 

Jupiter,  a  prolate  spheroid,  and 

why 62 

Jupiter's  belts        368 

Jupiter,  satellites  of 367 


Kaleidoscope 22i 

Kepler 337 

"  laws  of  ....  337,350,352 
Kepler's  opinion  of  the  number  of 

comets 380 

Klinkenfues  . 381 

Knee-joint 96 


Ladder  a  lever   .    .       77 

Lakes,  why  more  difficult  to  swim 

in 126 

Lamp,  defects  of,  how  remedied  .112 
Lamps,  why  they  will  not  burn  .  Ill 
Lamp,  wick  of,  bow  it  supplies  the 

flame Ill 

Lantanium 20 

Latent  heat 187 

Lathes 80 

Law,  Bode's .342 

Law,  fundamental  of  mechanics, 
pyronomics,  acoustics  and  op- 
tics   49 

Law,  Mariotte's 142 

"    of  falling  bodies 51 

Laws  >j{  heat -185 

"     of  reflected  sound    .    .    .  •.  178 
Laws  of  reflection  from  concave 

mirrors .  226, 227 

Law  of  the  heavenly  bodies     .    .  340 

Lead 20 

"     not  ductile 31 

"     why  heavy 34 

Le  Verrier 371 

Leap-year 3!>6 

Leaves  of  a  wheel  ...  .84 

Length 23 

Lens,  axis  of  a    ....  23? 


456 


INDEX. 


Lene,  ooicavo-convex    ....      233 

convex  as  a  burning-glass  .  235 

"     double  concave     .    .    .    .    .233 

"     double  convex 233 

Lenses 232,  482 

Lens,  effect  of  how  estimated  .    .  234 
"     focal  distance  of  a  .    .    .    .234 

Lenses  in  spectacles 230 

Lens,  single  concave 233 

"     single  convex 233 

"     the  crystalline 237 

Level,  how  ascertained 113 

"      or  equilibrium  of  fluids     .  110 

Levels,  spirit  or  water 113 

Lever 93 

"      advantage  in  use  of  ...    73 

Lever,  force  of  the,  on  what  de- 
pendent     70 

Lever,  how  used 72 

k<      kinds  of 72 

"      many  forms  of  the  .    ...    75 

"       of  first  kind 73 

"      of  second  kind 7(5 

«•      of  third  kind 78 

««      perpetual,  the 80 

Lever,  power  of  not  dependent  on 
its  shape 76 

Lever,  principle  of  the      ....    71 
"       the  bent 76 

Lever,  things  to  be  considered  in 
the 72 

Leyden-jar 263 

"          how  charged   ....  271 

Leyden-jar,  how  discharged  silent- 
ly   "  ....  265 

Light,  aberration  of 384 

"       absorbed  by  all  bodies  .    .217 

"       beam  ol       213 

"       color  of   ....  251,  252,  438 

Light,  corpuscular  and  undulatory 
theories  of 211,  431 

Light,  heat  and  chemical  action 
of 254 

Light,  how  projected 213 

Light,    intensity   of,   law    of  de- 
crease    212 

Light,  passing  into  different  medi- 
ums    230 

Light,  polarization  of   .....  256 

"       reflected 215 

"             "         laws  of     ....  216 
*«       reflection  of 211 

l/ght,  Sir  Isaac  Newton's  opinion 
of  .    .    .    .  • 211 

Light,  theories  of 211 

Light,  thermal,  chemical  and  non- 

tfleotn.«.f     ......  2&C 


Light,  velocity  of  .    .  41 

"       zodiacal 3GC 

Lightning,  how  caused 278 

Lightning-rods 265 

"             by  whom  invented  281 
Lightning-rods,  the  best,  how  con- 
structed     280 

Lime 21 

Lime,  carbonate  of 21 

Linen  garments,  why  cool    .    .    .  189 

Line  of  direction 66 

Liquid,  how  it  differs  from  a  fluid  109 
Liquids  have  a  slight  degree  of 

cohesion 109 

Liquids  not  easily  compressed     .    2S/ 
Liquid,   quantity   of    discharged 

from  an  orifice 129 

Lithium 20 

Load-stone 298 

Locomotive  steam-engine     .    .    .  208 

Looking-glasses 221 

Looking-glass,  length  of  to  reflect 

the  whole  person 223 

Lucifer 363 

Luminous  bodies 210 

Lutetia  339 

M. 

Machine 71 

Machinery,  propelled  by  electrici- 
ty   278 

Machine,  Attwood's 52 

Machines,  velocity  of,  how  regu- 
lated      106 

Magazine,  magnetic .307 

Magdeburgh  cups  .    .    .    .    .    ...  157 

Magnesia 21 

"         carbonate  of     ....    21 

Magnesium 20 

Magnet,  attraction  and  repulsion 

of 300,301 

Magnet,  attractive  power  of,  where 

greatest    300 

Magnet,  broken .  302 

Magnet   communicates   its  prop- 
erties     301 

Magnet,  dipping  of  a    .....  303 
"       effect  of  heat  upon    .    .  302 
Magnet,   horse-shoe    or   U,   how 

armed   .    .    .    .' 308 

Magnetic  influence,  all  bodies  sus- 
ceptible of .  301 

Magnetic  magazine 307 

Magnetic  needle     .......  304 

Magnet,  keeper  of  a  .        .    .  302,  SOS 
Magnet,  1'iMj.crtifg  of  .      29V 


457 


Magr«tic  poles    ....  .  300 

"         power  on  surface      .    .  302 

Magnetism 1".    \8,  298 

Magnetism    and    electricity     re- 
semblances of 302 

Magnet,  modes  of  supporting  .  .  300 
Magnetic  poles,  where  strongest  304 
Magnet,  north  and  south  poles  of, 

where  most  powerful     ....  306 
Magneto-electricity  .    .    .    .   1*,332 
Magneto-electricity,  most  power- 
ful effects  of,  how  obtained  .    .332 
Magneto-electric  machine    .    .    .  533 

Magnet,  polarity  of 299 

Magnet,  poles  of  changed  by  elei 

tricity JQ3 

Magnet,  powers  of,  how  increased  30 \ 

"      kinds  of 29? 

"      artificial,  how  made. 306,  30\ 

"       the  receiving 32* 

"       U  or  horse-shoe  ....  30] 

"      variation  of   .  303,  304  note 

Magnitude,  centre  of    ...     59,  66 

Main-spring  of  a  watch    .    .  104, 107 

Major  third      184 

Malleability 27,31 

Malleability   dependent    on  tem- 
perature  . 31 

Manganese 20 

Marco  Paolo 306 

Mariner's  compass 304 

Mariner's   compass,    inventor   of 

the 306 

Mariner's  compass,  needle  of,  how 

placed  305 

Mariner's  compass,  how  mounted  305 
"  "         points  of  the  305 

Mariotte's  law 142 

Mars 366 

Massila 339 

Materials,  strength  of 95 

Materials  which  compose  the  crust 

of  the  earth 20 

Materials,  tenacity  of 32 

Matter,  attractive 34 

"       definition  of 19 

"       essential  properties  of    .    21 
"       gaseous  form  of   ....    19 
.Matter,    its    different    states    or 

forms 19 

Matter,  liquid  form  of 19 

Matter,  quantity  of,  how  estimat- 
ed         40 

Materiality  of  air 162 

Matter,  solid  form  of 19 

Mechanical  agency  of  fluids     .    .  1  18 
"  equivalent 58 


Mechanical  operations  always  at- 
tended by  heat .188 

Mechanical  paradox 68 

"  power 70 

"  poAvers    ......    71 

Mechanical  powers,  enumeration 

of  the    ...    ,9 72 

Mechanical  powers,  on  what  prin- 
ciple constructed 71 

Mechanical  powers,  principal  law 

of  the 89 

Mechanical  powers,  reducible  to 

three  classes 72 

Mechanical   properties   of  gases, 

vapors,  &G. 139 

Mechanics 17,41,403 

"         fundamental  law  of .  71,  91 
118 

Mechanics,  fundamental   law   of, 
its    application   to   hydrostatic 

pressure 119,  413 

Media 97,  229 

Medium 97 

Mediums 97,  229 

Medium  in  optics 230 

Melpomene 339 

Meniscus      233 

Mercurial  pendulum 103 

"         tube 160 

Mercury 20 

"        ths planet,  transit  of    .    .  363 
Mercury,  the  planet,  why  not  often 

seen 362 

Metallic  points 265 

Metals,  good  conductors  of  heat  .  190 

"       names  of  the 20 

Metals,  order  of  their  conducting 

power  of  heat 190 

Metals,  tenacity  cf 32 

Meteoric  stones  .    , 387 

Meteoric   stones,   J*-.   Brewster's 

opinion  of 367 

Metes 339 

Mica 21 

Microscope,  a  double 24S 

"  a  single 242 

Microscope,   compound   nrp^nify- 

ing  power  of,  how  ascertained    244 
Microscope,  magnifying  power  of, 

how  ascertained 244 

Microscope,  the  solar 244 

Microscope,  the  solar,  3it,guify'ug 

power  of 241 

Microscopes,     what      hjvve      ti^ 

greatest  magnifying  pow«r  .       2-1 r, 
Milk,   why    aiusctjd     by 
and  lightufcg . 


458 


INDEX. 


Milky-way .  383  I 

Minor  thirl 134 

Mirror 221  j 

"      concave 222  j 

"      convex 222  ; 

"      plain 221 

Mirrors  of  half  the  he  ght  show  a 
whole-length  figure    .    .        .    .2171 

Mirrors  reverse  all  images       .    .  222 
"        use  of  glass  in  .    .        .    .  221 

Miscellaneous    Ciperiments   with 
air 166  i 

Mobility 27  j 

Molybdenum 20 ! 

Momenta 50  j 

Momentum 41,50! 

Momentum  of  a  body,  how  ascer- 
tained    50 ; 

Monochord 182 

Moon .  386 

"     as  cause  of  tides 391 

"     as  seen  through  a  telesoope  389 

Moon,  common  errors  in  respect 
to  the 386 

Mooa,  density  of  the 387 

"      di  Here  nee  in  daily  rising    389 

"      gibbous 388 

"      harvest  and  hunter's    .    .  389 

"      horned     388 

"      in  quadrature 388 

Moon-light,  objects  seen  by,  why 
faint 217  I 

Moon,  surface  of  the 386  i 

"       uninhabitable 3(i4  | 

Morienne 144  j 

Morse's  telegraph 320! 

"       telegraphic  alphabet  .    .  323  | 

Motion      41 

Motion,  accelerated,  retarded  and 
uniform 44 

Motion,  axis  of 59 

"        centre  of 59 

Motion,  how  transmitted  by  hy- 
drostatic pressure  121 

Motion,  incident  and  reflected    .    47 

Motion  impelled  by  two  or  more 
forces 55 

Motion  of  the   heavenly  bodies, 
cause  of  the 34 

Motion,  perpetual 45,  407 

"        regulators  of 100 

"        reversed 83 

Motion,  slow  or  rapid  proluced  at 
pleasure  by  machinery  ....    84 

Motion,  when  imperceptible    .    .  220 

Moving   p<  wer  in  machines,  how 
stopped     .        8fi 


Mountain,  how  burst  by  hydro- 
static pressure 12$ 

Musical  scale 183 

"        sounds   181 

Multiplier,  electro-magnetic  .  .313 

Multiplying-glass 235 

Musical  chord,  how  produced  .  .  182 
Musical  instruments,  why  affected 

by  the  weather 182 

Macic  of  a  choir  dependent  on  the 

uniform  velocity  of  sound  .  .176 
Music  of  strings,  how  caused  .  .  181 
Mutual  attraction  .  .  34 


N. 

Natural  Philosophy,  definition  of  17 

Neap  tides 391 

Needle,  the  magnetic 304 

Needle,  how  placed  in  a  mariner's 

compass 305 

Negative  electricity  ....  259,  202 
"  (galvanic)  pole  ...  287 

Neptune 371 

Newcomen  and  Savary's  steaiu- 

engine 197 

Newton,  Sir  Isaac  ....  23,  337 
Newton,  Sir  Isaac,  discovery  of  - 

gravitation 100 

Newton's  discoveries,  on  what 

based 352 

Newton's  (Sir  Isaac)  opinion  of 

light 211 

Newton,  Sir  Isaac's,  opinion  of  the 

earth's  compressibility  ...  29 

Nickel 20,  298 

Niobium 20 

Nitrogen 20 

Non-conductors  .  .  .  15?,  2l)fl 

Non-electrics 258,  260 

Nut  and  screw  .  95 


Oars,  on  what  principle  construct 

ed .  .71 

Object,  apparent  size  of,  on  what 

dependent  220 

Objects,  when  invisible  .  218,  220 
Octave 184 

"  how  produced 182 

CErsted's  discoveries  In  electro 

magnetism 108 

Oil,  effects  of  in  smoothing  the 

surface  of  water ^31 

Oil,  glutinous  matter  in  .  .  Ill 
Ji'-ujillB  .  US 


INDEX. 


459 


Oil,  why  it  fiVata 39 

Giber's,  Dr.,  opinion  on  lunacy  .  386 

Opaque  bodies 217 

Opera-glasses 249 

Opposition 350 

Optical  paradox  212 

Optis-nerve 237,  240 

Optics 17,  210 

"  definitiun  uf 18 

Oracles  of  Delphi,  Ephesus,  Ac.  .  180 

Orbit,  meaning  of .  340 

Orbits  of  the  planets,  inclination 

of 347 

Orbits  of  the  planets,  not  circular  343 

Otto  Guericke 158 

"  Out  of  beat,"  meaning  of .  .  .104 

Ovorshot-wheel 82 

Osmium  ....  20 

Oxyde  of  iron  .  . 21 

Oxygen 20 


Pails,    why    two   can   be   carried 
more  easily  than  one     ....    69 

Palladium 20 

Pallas 339 

Parabola 62,  341 

Parachute 38 

Paradox 118 

acoustic 177 

hydrostatic 118 

mechanical 68 

optical 212 

pneumatic 169 

Paradox, optical,  pneumatic,  acous- 
tic, Ac.,  no  paradox      .    .  212  note 

Parallax 385 

Parallel  motion,  appendages  for    200 

Parallelogram 48 

Parthenope 839 

Pascal 144 

Pelupium 20 

Pendulum 100 

Pendulum,  cause  of  slowness  and 

rapidity  of  vibrations    .    .    .    .102 
Pendulums,     continuous     motion 

of,  how  preserved 103 

Pendulum,    how    lengthened    or 

shortened 102 

Pendulum,  how  to  be  suspended    10:i 
"  its  motion,  how  caused  101 

Pendulums,  length  of,  proportion 

of 103 

Pondulu'Ji,  length   of  to   vibrate 
seconds 102 


Pendulum,  length   of  to    ei orate 

two  seconds 103 

Pendulum,  length  of  varies  with 

the  latitude 102 

Pendulums,  table  of  the    lengths 
of  to  beat  seconds  in  different 

latitudes 104 

Pendulum,  the  ballistic    .    .    .    .    6J 
"  the  gridiron    ....  10U 

"  the  mercurial  ....  103 

Pendulums,    to     what     variations 

subject 103 

Pendulum,  use  of  the  ball  of.  101  note 

Penumbra 394 

Percussion,  force  of 93 

Perigee 349 

Perihelion 349 

Permanent  magnets 301 

Perpendicular 48 

Perpetual  lever  .80 

".         motion 45 

Perpetual  motion,  approximation 

to 288 

Phocea      339 

Phosphor 363 

Phosphorus 20 

Photography 257 

Physical  spectra 228 

Physics,  definition  of 17 

Piazzi   .        343 

Pincers 75 

Pinions 83 

Pipes,  tones  of,  on  what  dependent  181 

Pivots 8] 

Plane,  the  inclined 90 

Planet,  meaning  of 339 

Planet   and    star,   difference   be- 
tween     339 

Planets,  characters  by  which  they 

are  represented 34fi 

Planets,  inferior  and  superior  .    .  343 

"      minor 339,  367 

"           "     how  discovered    .  342 
Planets,  minor,  by  whom  discov- 
ered   343 

Planets,  minor,  size  of 344 

"      names  of  the   ...  338,  339 
Planets,  relative  appearance   of, 
as  seen  through  a  telescope     .  372 

Planets,  the  primary 338 

Planets,  when  in  a  particular  con- 
stellation       349 

Platinum 20 

Platinum,  both  ductile  and  malle- 
able   31,32 

Plough,  constellation  of  the       ",  398 
Pluuib-lu-e 3" 


460 


INDEX. 


Pneumatics 17    18,  138 

Pneumatic  balloon 1(51 

Pneumatic  paradox 169 

"  shower-bath     ....  166 

"  scales 160 

Pointers 3H8 

Poker 75 

Polarity 299 

boreal  and  austral  .    .  ,.  302 

Polarization  of  light 256 

Polar  or  pole  star 384,398 

Poles,  magnetic 300,304 

Poles,  magnetic,  where  strongest  304 

Ponderable  agents 18 

Pope  Callixtus  and  the  comet  of 

Halley 378 

Pores 28 

Porosity 27,  28 

Positive  electricity    ....  259,  262 
Positive  (galvanic)  polo   ....  287 

Potash 21 

Potassium 20 

Power 72,  405 

Power,  how  gained  by  use  of  the 

lever 76 

Power,  how  to  be  understood   73,  405 

Powers,  mechanical 70,  72 

Power  that  acts 7 

Power,  weight  and  velocity,  pro- 
portion of 90 

Precession  of  the  equinoxes     .    .  397 
Press,  Bramah's  hydrostatic  121,  415 
Presses,  screws  applied  to    ...    95 
Pressure  at  any  depth,  how  esti- 
mated  115 

Pressure,  fluid,  law  of 115 

Pressure,   hydrostatic,   as  a  me- 
chanical power 121 

Pressure,  hydrostatic,  caused  by 
height,  not  by  quantity    .    .    .  119 

Pressure  of  fluids 114 

Pressure  of  fluids  in  proportion  to 

height  of  column 129 

Pressure  of  the  air     ....  141, 162 
"       of  water  at  great  depths  109 
Pressure  on  hydrostatic  bellows, 

how  estimated 119 

Primary  planets 338 

Principle  of  all  machines     ...    72 
Principle  of  the  mechanical  pow- 
ers      71 

Prism 252 

Projectiles 62 

Projectile,  random  of 65 

Projection,  force  of 62 

Projection,  force  of,  has   10  effect 
on  gravity    . 64 


Propeller      ......  204 

Properties,  essential  aud  acciden- 
tal, of  matter  ........    21 

Properties,  essential  and  unesssu- 
tial    ............    23 

Prussian  blue      ......          3  '27 

Psyche      .........          539 

Ptolemy    ..........    .536 

Pulley  ...........    8C 

"      fixed  and  movable    ...    86 
"      fixed,  use  of  ......    87 

Pulleys,  mechanical  principle  of 
same  as  that  of  levers  ....    88 

Pulley,    movable,   how   it   differs 
from  a  fixed     ........    87 

Pulley,  movable,  principle  of  the  89 
Pulley,  power  of,  how  ascertained  88 
Pulleys,  practical  use  of  ....    89 

Pump,  the  chain     .......  131 

"      the  common,  for  water  .    .152 
««      the  forcing     ......  153 

«l      the  air     ........  154 

Pupil    ..........  237,  238 

Pyramid,  why  the  firmest  of  struc- 
tures    ...........    68 

Pyrometer  ..........  193 

"          Wedgewood's  ...      193 
Pyronomics  ....   17,  18,  185,  187 

Pythagoras  .......  336 


Q. 


.    .388 


Quadrature.    .        ... 

Quartz  .....    .......    ^1 

Questions  for  solution    36,  42,  43,  50 

53,  54,  78,  86,  90,  96,  106,  116,  127, 

184 


U. 


Radiation  of  heat 190 

Radii 48 

Radius 48 

"       vector 350 

Rain,  how  formed  .  .  .  121,150,  186 
Rainbow,  how  produced  ....  255 
Ram,  the  battering 105 

"  the  hydraulic 133 

Random  of  a  projectile  ...»  65 

Rarefaction  .  .- 140 

Rarefied 140 

Rarity  .........  .  27, 28 

Ray  of  light 21 1 

Rays  of  light  absorbed  .  .  .  .  215 

"  "  converging  .  .  .  212 
Rays,  converging  and  diverging, 

laws  of     ,  .  22" 


INDEX 


461 


Rays  of  light,  diverging  .        .    .  ^i2 
Rays  of  light  from  terrestrial  ob- 
jects .    , 213 

Reader,  The  Rhetorical  ....  180 
Reaumur's  thermometer  .  .  .  149 

Receiver 154 

Rectangle 48 

Rectilinear   motion  converted  to 

circular 81 

Reflected  motion 47 

Reflecting  substances    .....  211 
"          telescope  .    .  246,  249,  444 

Refraction 229 

Refracting  substances 211 

"  telescope     .    .    .  246,  443 

Refrangibility 230 

Registering  apparatus  of  the  tel- 
egraph   322 

Regulators  of  motion 100 

Rein,  F.  C.,  hearing  trumpets  or 

cornets 178  note 

Repulsion f.   .    .    .    28 

Resinous  electricity 262 

Resistance 41 

Resistance  of  a  medium,  to  what 

proportioned 97 

Resistance  of  the  air 38 

"          to  be  overcome  ...    71 

Resultant 58 

"         motion 57 

"         of  two  forces     ....    56 
Resultant  of  two  forces,  how  de- 
scribed       58 

Retarded  motion  of  bodies  pro- 
jected upwards 54 

Retina ...  237,  240 

Reversed  motion        83 

Revolving-jet 163 

Revolution  of  the  planets,  length 

of 341 

Rhetorical  Reader     .......  180 

Rhodium 20 

Rhodes,  siege  of         105 

Rifles,  how  tested 63 

Rivers,  how  formed 124 

Rivers,  why  difficult  to  swim  in  .  126 
Rivulets,  how  formed  .  .  .  124,  136 
Roads,  inclined  planes  ....  91 

Rolling  friction 98 

Romans,  the    ancient,   how   they 
conveyed  water       ......  137 

Rope-dancer,  how  enabled  to  per- 
form his  feats 67 

Ropes,  strength   of,  on  what  de- 
pendent     100 

Rosse's  telescope 251,444 

Rotation,  electro-magnetic  .        .  313 


Rudders,  on  what   principle  con- 
structed    77 

Rules  relating  to  musical  strings  184 
Rules  by   which    changes  of'  the 
weather  may  be  prognosticated 
by  means  of  the  barometer  .    .  147 
Rules  relating  to  musical  pipes  .  184 
Rush's  Treatise  on  the  Voice  .    .  180 
Ruthenium 20 

S. 

Safety-valve 199 

Sagacity  of  animals 92 

Sap,  ascent  of,  to  what  due  .112 
Satellites,  general  law  of  ....  370 

Saturn 368 

Saturn's  rings 368 

Scales  for  ascertaining  specific 

gravity 126 

Scale,  -the  musical 183 

Scales,  the  pneumatic 16C 

Schorl 21 

Science  of  harmony,  on  what 

founded 1S2 

Scissors 75 

Sclerotica .'237 

Screw 93 

a  compound  power  ....  94 
advantage  of  the  ....  94 
convex  and  concave  ...  94 
power  of,  how  estimated  .  94 

Hunter's 95 

of  Archimedes 132 

uses  of  the 95 

"Sea-Eagle,"  experiment  made 

on  board  of  the 109 

Seasons,  cause  of  the 350 

"  explanation  of  the  cause  355, 
356 
Sea-water,  cause  of  its  increased 

specific  gravity 12G 

Seebeck,  Professor,  discoveries  of 

in  thermo-electricity  ....  334 

Selenium 20 

Serpentine 21 

Shadow 213 

Shadows,  darkest,  how  produced  214 
Shadows  from  several  luminous 

bodies 215 

Shadows,  increasing  and  diminish 

ing 214 

Shadow  of  a  spherical  body,  form 

of 2U 

Shadows,  why  of  different  degreed 

of  darkness 213 

Sli  aft  .  81 


462 


Shet herds,  balancing  of  in  south 

of  Frame GT 

Ships,  on  what  principle  they  float  123 

Sidereal  time 3(J6 

"      year 39G 

Silence  produced  by  two  sound?    177 

Silica 20,  21 

Silver   best  conductor  of  heat  .    .  IS  ' 

"Simple  motion 55 

Sidereal  year,  how  measured  .    .  397 
Signal-key   of  the    electric   tele- 
graph     322 

Signs  of  the  zodiac 346 

Signs  used  in  almanacs     ....  389 

Silurus  electricus    .    . 282 

Silver 20 

Siphon      132 

Siphon,  equilibrium  of  fluids  ex- 
emplified by  means  of  the    .    .  133 
Siphon,  experiments  with  the      .  167 
"       principle  of  the    .    .    .    .133 

Sky,  why  blue 253 

Slate  formations  in  Bohemia  .    .    23 
Slaves  in   West  Indies,  how  they 

steal  rum 122 

Steel,  how  made  brittle    ....    30 

Sliding  friction 98 

Soiee's  battery 290 

Smoke,  why  it  ascends 39 

Snow,  how  formed      ....  124, 150 
"      how  it  differs  from  hail     .  I'z4 
Snow  and  ice,  how  made  to  melt 

rapidly 191 

Snuffers 75 

Soap-bubble,  thickest  part  of  .    .    23 

Soda      .    .    . 21 

Sodium 20 

Solar  microscope 214 

Solar  system,  account  of  the  337,338 

«     time 396 

"     year,  how  measured    .396,397 

Solstices 358 

Sonorous  bodies 174 

Sonorous   property  of  bodies,  to 

what  due 175 

Sound 174 

Sound   affected  by  the  furniture 

of  a  room 179 

Sound,  by  what  laws  reflected  .    .178 
Sound,  by  what  reflected  and  dis- 
persed   179 

Sound,  focus  of 179 

Round,  how  communicated  most 

rapidly         175 

Soui.d  of  the  human  voice  .  .  .  179 
"  of  strings,  ho*y  causoc1  .  .181 
*  rapidity  of .  ...  .  .  176 


Sounds,  distance    to   which   the^ 
may  be  conveyed   ......  17^ 

Sounds,  musical .  18? 

"       producing  silence     .    .    .  177 

Sound,  velocity  of     .    .    .    .176 note 

Sounds,  what  pleasing  to  tne  ear  183 

"       when  loudest    .        ...  174 

Sources  of  heat 187 

Space    ............    41 

"     how  estimated 43 

Speaking-trumpets 178 

Specific  gravity  .    .  40,  126  note,  410 
Specific  gravity  of  bodies,  how  as- 
certained   125,  127,  417 

Specific  gravity,  scalos  'or  ascer- 
taining      126 

Specific  gravity,  standard  of   .    .  123 
"     gravities,  table  of  .   .   .124 

Sphericity,  centre  of 37 

Spectacles .   .  236 

Spectrum  of  a  prism 254 

Spherical  aberration 247 

Spherical  body,  how  made  to  roll 

down  a  s.'ope 68 

Spider's  web ?3 

Spiral  tube 274 

Spirit  level 113 

Spirit  or  water  level,  with  what 

filled 113 

Spots  in  the  tun 304 

Sportsman  aiming  at  a  bird  ...  57 
Spring,  how  high  it  will  rise  .  .  137 
Springs,  how  formed  .....  136 
Spring-tides  ....  ,  .  391 

Spur-gear 84 

Spur-wheel 84 

Square ....    48 

Square  rods,  why  bi  tter  than  round 

as  conductors  of  electricity  .    .  279 
Standard  of  specific  gravity     .    .  123 

Stars,  distance  of  the 382 

Stars,   distance   of  the,  Sir  John 

HerscheFs  opinion  of    ....  383 
Stars,   how     distinguished     from 

planets .  339 

Stars,  the  fixed 381 

Stars,  why  not  seen   in  the  day- 
time   363 

Stars,  why  not  seen  in  their  true 

place .  384 

Statics 17,18 

Stationary  steam-engine  ....  209 

Steam 195 

Steamboats 205 

Steam,  cause  of  the  ascent  of  .   .124 

"      dry  and  invisible  ....  19G 

Steam  engine  applied  to  boat*    .  103 


INDEX. 


463 


Steam-engine,  power  of,  how  esti-       ]  Sun,  planets  and  stars,  inhabited  359 


mated 199 

Steam-engine,  the      ......  1UG 

"  improvers  of  the  .  2UO 

Steam-engine,  Neweoinen  and  Sa- 
vary's 197 

Sleaiu-engine,  Watts'  double  act- 
ing, condensing 197 

S.eam-engine,    Watts'    improve- 


Sun,  rei  appearance  of  the,  how 

caused  253 

Sun's  heat,  effect  of  on  the  earth.150 

Superior  conjunction 349 

"       planets 343 

ourinam  eel 282 

Suspension  of  action 85 

Synchronous  tickings  of  a  clock  .  104 


uients  of  the 197  j  Syracuse,  King  of,  employs  Ar- 


Steain-engine,  the  locomotive 


204, 
208 


Steam-engine,  the  stationary  .    .  209 
Steam-engine,  Tufts'  stationary    207 
Steam,  foundation  of  its  applica- 
tion to  machinery 30 

Steam,  how  condensed  into  water  195 


how  made  to  act 


196 


chimedes  to  detect  the  adultera- 
tion of  a  crown 127  note 

.    .  188 
156, 163 


Steam,    on    what   its   mechanical 

agency  depends  195 

Bteatn,  pressure   of,  on   what   de- 
pendent     195 

Steam-ship 203 

Steam,  space  occupied  by    ...  196 

"      temperature  of 195 

"      why  it  ascends '39 

Steatite 21 

Steelyards 75 

"          how  to  be  used       .    .    74 
Steelyards,  mechanical  principle 

of  the 73 

Stereo-electric  current 334 

Stethoscope     175 

Still 194 

Stilts  used  in  south  of  France     .    67 

Straight  jet 163 

Strata  of  the  earth 20 

Stream,  velocity  of,  how  measured  130 
Strings,  musical   sounds  of,  how 

produced 181 

.Strings,    musical   quality    of  the 

sounds  of 181 

Strontium 20 

ftruve's  opinion  of  the  distance 

of  the  stars 382 

Substance,  heterogeneous     ...    19 
"  homogeneous  ....    19 

Sucker 100 

Sulphate  of  copper  battery  .    .    .  292 
Sulphate  of  copper  battery  (pro- 
tected)       293 

Sulphur 20 

Sun,  as  cause  of  tides 391 

"    as  viewed  from  the  planets     360 

"     its  size,  &c 359 

Sun,  moon  aud  planets,  relative 
•use  of  the 343 


Syringes  for  striking  fire 
Syringe,  the  condensing 

T. 
Table  of  specific  gravities 


.  124 
Table  of  the  lengths  of  pendulums  104 

"     of  velocities 42 

Tackle  and  fall 89 

Talc 21 

Tangent 48, 60 

Tantalus 133  note 

Tantalus'  cup     133 

Tantalize,  origin  of  the  word  .    .133 

Tapestry  of  Bayeux 380 

Tea-pots,  why  they  have  handles 

of  wood 190 

Teeth 83 

Telegraph,  atmospheric    .    .    .    .331 

"          Bain's 326 

"          electric,  history  of  the  329 
"          electro-magnetic     .    .  319 
Telegraph,  electro-magnetic,  rep- 
resentation of  the 323 

Telegraph,  electric,  principles  of 

its  construction 320 

Telegraph,  House's  printing        .  328 

Telegraphic  battery 321 

Telegraph,  meaning  of  .        .  319  note 

Telescopes 246 

Telescope,  achromatic  .  .  .  247,  442 
"  Cassegrainian  .  .  .  250 
"  day  and  night  .  .  .  248 
"  Gregorian  .  .  .  250,  443 

liersehePs 251 

"  "          power  of     .337 

"          Lord  Rosse's    .    .  251,  444 
"  reflecting  ....  240,  444 

««  refracting    ...  24C,  443 

"          simplest  form  of  the  .  247 

Tellurium 20 

Tenacity 27,32 

"        of  cords 32 

"        of  the  metals      .   .   .    .    3¥ 
"        of  metals,  how  iiioroo^Hl  3? 


464 


INDEX. 


Tenacity  of  various  substances   .    32  |  Trumet,  speaking     .    .    . 

fender  of  a  steam-engine     .    .    .  204  I  Tubes,  capillary 

^erbium 20 1       "      inercuru.1 

Terrestrial  gravity 34  i  Tufts'  stationary  steain-engice 

pi „!    «.&?,,«  ±  „   „  .£*  i ;  .„!,  A  ne/>      m         _ 

Tungsten 


Thermal  effects  of  light    ....  256 

Thermometer 149 

Celsius' 149 

"  Delisle's 149 

"  Fahrenheit's   .    .    .149 

Thermometer,  on  what   principle 

constructed 29 

Thermometer,  Reaumur's     .    .    .  149 

Thermo-electric 334 

"  batteries.    .    .    .335 

Thermo-electricity     ....  2CO,  334 

Thetis 339 

Thorium 20 

Threads  of  a  screw 93 

Thunder-clouds,  distance  of,  how 

measured 177 

Thunder-house 277 

Thunder-storm,  safest  position  in.2«l 

Tides 390 

"     neap  and  spring 391 

Time,  apparent  and  true,  differ- 
ence between 397 

Time  as  kept  by  clock  and  by  the 

sun 397 

Time  employed  in  the  ascent  and 
descent  of  a  body  equal    ...    54 

Time,  how  estimated 43 

"      sidereal  and  solar  ....  396 
Time  of  ascent  and  descent  of  a 

body 45 

Tin 20 

Tin  and  copper*  sonorous  proper- 
ties of 30 

Tin,  not  ductile 31 

TLs.-'ue  figure 270 

Titanium 20 

Toggle-joint 96 

"  operation  of  the   .    .    97 

Tones  of  the  voice,  how  varied    .  180 

Tonic 183 

Tonnage  of  vessels,  how  estimated  123 

Torpedo    .    .    ." 282 

Torricelli 143 

Torricellian  vacuum      .....  143 
Towns  and  fortifications,  attacks 

on 

Transfer  of  fluids 167 

Transit  of  Mercury  and  Venus    .  303 

Translucent  bodies 211 

Transparent  bodies 211 

Tropic 356 

Trumpet  . 178 

Titiujpets,  hearing 178 


Tycho  Brahe   . 


17N 

111 

160 

20'i 

41 

20 

336 


U. 


Umbrella,  use  of  in  leaping  fioiu 

high  places 38 

Undershot  wheel 82 

Undulations  of  light 211 

"  of  water,  effects  of  .  131 

Undulatory  theory  of  light  .    .    .  211 
Universal  discharger  ...  272 

Urania 339 

Uranium 20 

Uranus     369 

"      moons  of 370 

Ursa  Major 398 

Unit  of  Heat 4.80 

Unit  of  Work 404 

V. 
Vacuum 98,  143 

Vacuum,  a  perfect,  not  to  be  pro- 
cured by  means  of  the  air-pump  156 
Vacuum,  the  Torricellian      .    .    .  143 

Valve 152 

Vanadium 20 

Vapor,  cause  jf  ascent  of  ....  12-1 

Vapors 139 

Vegetables,  why  white  or  yellow 

when  growing  in  dark  places  .  256 
Vehicle  in  motion,  cause  of  acci- 
dents from 25 

Velocities,  table  of 42 

Velocity 41,71 

"       absolute  and  relative  .    .    42 

"       how  estimated 42 

Velocity  of  balls  thrown  by  gun- 
powder       63 

Velocity  of  light  and  of  the  elec- 
tric fluid 40 

Velocity  of  parts  of  a  body,  how 

diminished 60 

j  Velocity  of  sound  .    .    .176  and  note 
63  j  Velocity     of     sound,      distances 

measured  by  the 177 

Velocity  of  sound,  experiments  of 

Arago,  Gay  Lussac  and  others    170 
Velocity  of  a  stream   how  meas- 
ured   130 

Velocity    of   the    surface    of     a 
stream,  greatest I2i! 


INDEX. 


405 


Velocity   required    in    machines, 

bow  regulated 106 

Ventriloquism 180 

Venus 363 

"      transit  of 3C3 

Venus,  why  never   seen  late  at 

night 363 

Vertical  line 37 

Vesicular    form    of  matter,  defi- 
nition of 19 

Vespasian,  battering-ram  of    ,    .  108 

Vesper  .    . 363 

Vessels,   tonnage    of,   how    esti- 
mated    123 

Vesta 339 

Vision,  angle  of 219 

Victoria 339 

Vitreous  electricity 262 

Vitreous  humor 237,239 

Vitriol,  effects  of  on  water  .    .    .  187 
Voice,  Dr.  Rush's  Treatise  on  the  180 

"       sound  of  the 179 

Voice,  the  human,  imitative  pow- 
er of  the  180 

Voice,  tones  of  the,  how  varied  .  180 

Voltaic  battery 289 

"       electricity     .    .    .      259,283 
pile     .    .' 288 


War,  how  it  has  beeu  elevated  to 

a  science 63 

Warmth  of  clothing,  cause  of  .    .  189 
Watch,  how  it  differs  from  a  clock. 104 

"      how  regulated 105 

"      moving  power  of  a  .    .    .  104 

Water 21 

Water,  converted  into  steam,  space 

occupied  by 30 

Water,  distilled,  the  standard  of 

specific  gravity .123 

Water,  elasticity  and   compressi- 
bility of 24 

Water  expands  when  freezing  .    .192 
Water-fowl,  buoyancy  of  .    .    .    .123 
Water  frozen  under  the  air-pump.  169 
Water,  how  applied  to  move  ma- 
chinery      83,  419 

Water,  how  converted  into  steam. 195 
Water,  how  high  raised  by  means 

of  common  pump 153 

Water,  how  much  diminished  in 

bulk  by  pressure    ....    29,  412 
Water,  instruments  for  raising  .  131 

Water-level 113 

Water,  motion  of, how  retarded  .  129 


Water,  not  destitute  ol  compress- 
ibility   109 

Water,  of  what  composed     .    .        20 
Water,    pressure     of     at     great 

depths 109, 116 

Water,  pressure  of  at  any  depth, 

how  estimated 115 

Water,   pressure    of   at   diflereat 

depths  115 

Water-pump 152 

Water-spouts 172 

Water,  weight  of  a  cubic  foot  of  .  126  , 
"      weight  of  a  cubic  inch  of  115 
Water  when  falling,  why  less  in- 
jurious than  ice 114 

Water,  when  perfectly  pure     .    .  124 
Water,  why  it  appears  more  shal- 
low than  it  is^ 231 

Water-wheels 81,419 

"  most  powerful     .    .    82 

Watson,   Dr.,  experiment   of,   to 
show  degree  of  evaporation  .    .150 

Watt,  James 106 

Watt,  James,  his   improvements 

of  the  steam-engine 19? 

Waves,  how  caused 130 

Waves  of  light,  laws  of  212  note,  437 

Wedge 92 

"       advantage  of  the    ....    92 
Wedge,    effective    power    of.    on 

what  dependent 92 

WTedge,  power  of  the 92 

Wedges,  use  of 92 

Wedgewood's  pyrometer  ....  193 

Weight 34,72 

"       cause  of 34 

"       lifter 165 

Weight,  loss  of  in  bodies  weighed 

in  water 126 

Weight  of  any  body,  how  ascer- 
tained by  its  c'ibical  contents  .  125 
Weight  raised  by  wheel  and  axle, 

how  supported 79 

Weight,   what    bodies    have   the 

greatest 34 

Welding 31 

Wheel  and  axle 78 

"  "     advantage  of  .        79 

"  "     construction  of  .    79 

"  "     how  supported    .    81 

"  ««     principle  of  the  .    80 

Wheel,  escapement 104 

Wheels,  friction      . 99 

Wheels   in  machinery  acting  as 

levers 78 

Wheels,  large   and  small,  advan- 
tages of  each 85 


466 


INDEX. 


Wheels,  locked,  how  and  wo>  .  85 
Wheels  of  a  clock,  their  U30  101 

"       power  of 80 

"      size  of  limited  bv  iftat      85 
"       tires  of  how  secured    .      1(J3 
Wheels,  toothed,  method  ut  ascci 

taining  power  of 85 

Wheels,  use  of  on  roads  ....  85 
\Vheol  with  teeth,  of  three  kinds  34 

Whirlwinds 172 

Whispering-gallery 1/0 

Whispering-gallery  in  Newbury- 

port 179 

Whisper,  motion  of  a,  rapidity  of 

the 170 

White '251 

Whitefield 179 

Wick  of  a  lamp,  principle  of  the    111 

Width 23 

Wightuian's  apparatus  for  inertia  25 
William,  Duke  of  Normandy  .  .  380 
William  the  Conqueror  ....  380 
Winch  applied  to  wheel  and  axle  79 

"      double 80 

Wind    .    . 170 

Wind,  cause  of  the  different  direc- 
tions of  the 171 

Wind,  east,  cause  of  at  the  equa- 
tor          171 

•find    Instruments,  sound   of,  on 

what  iependent 181 

Wind*   juality  oi  the,  ho\\  affect- 
ed .  171 


Wind,  why  it  subsides  at  sunset  .  !TJ 

Windlass 80 

Windlass  and  capstan,  difierence 

between 80 

Wind-mill? 80 

Window,  where  the  hand  should 

be  applied  to  raise  .  .  .  .  77 
Wollaston,  experiments  of  ...  254 
Wooden  spoons  and  forks,  why 

preferred  for  ice 190 

Woollen  garments,  why  warm  .  189 
Worcester,  Marquis  of  ...  20« 
Worm  of  a  still  .  .  ...  19.5 
Work,  Unit  of, 404 

Y. 

Year 341 

Year,  leap 396 

Year,  sidereal  and  solar  .  .  .  396 
Yttrium  .  .  20 


Zodiac  .  .  345 


Zodiacal  light 360 

Zodiac,     constellations     of     the, 

change  of 347 

Zodiac,  signs  or  the 34< 

Zinc 20 

Zinc,  at  what  temperature  malle- 
able   S3 

Zirconium & 


/ 

'.    • 


YB  360CO 


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